Solved.

I finally got rid of the hanging shutdown/reboot by blacklisting the  i82975x_edac module,
adding a the line

blacklist i82975x_edac

in the file /etc/modprobe.d/blacklist.conf

After unsuccessfully trying out a lot of things and many reboots the final hint to my "solution" came in message  display by dmsg:

....
Sep 24 06:52:35 fish kernel: [    3.107842] usb 4-2: Manufacturer: DELL
Sep 24 06:52:35 fish kernel: [    3.126222] input: DELL DELL USB Laser Mouse as /devices/pci0000:00/0000:00:1d.2/usb4/4-2/4-2:1.0/input/input4
Sep 24 06:52:35 fish kernel: [    3.126438] hid-generic 0003:046D:C063.0002: input,hidraw1: USB HID v1.10 Mouse [DELL DELL USB Laser Mouse] on usb-0000:00:1d.2-2/input0
Sep 24 06:52:35 fish kernel: [    3.620790] random: nonblocking pool is initialized
Sep 24 06:52:35 fish kernel: [    6.801749] Adding 3906556k swap on /dev/sda3.  Priority:-1 extents:1 across:3906556k FS
Sep 24 06:52:35 fish kernel: [    6.952753] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
Sep 24 06:52:35 fish kernel: [    7.076382] systemd-udevd[290]: starting version 204
Sep 24 06:52:35 fish kernel: [    7.823881] intel_rng: Firmware space is locked read-only. If you can't or
Sep 24 06:52:35 fish kernel: [    7.823881] intel_rng: don't want to disable this in firmware setup, and if
Sep 24 06:52:35 fish kernel: [    7.823881] intel_rng: you are certain that your system has a functional
Sep 24 06:52:35 fish kernel: [    7.823881] intel_rng: RNG, try using the 'no_fwh_detect' option.
Sep 24 06:52:35 fish kernel: [    7.899351] EDAC MC: Ver: 3.0.0
Sep 24 06:52:35 fish kernel: [    7.951314] BUG: unable to handle kernel NULL pointer dereference at           (null)
Sep 24 06:52:35 fish kernel: [    7.951489] IP: [<ffffffffa00c97ee>] edac_create_sysfs_mci_device+0x23e/0x550 [edac_core]
Sep 24 06:52:35 fish kernel: [    7.951636] PGD 796d1067 PUD 796d2067 PMD 0 
Sep 24 06:52:35 fish kernel: [    7.951808] Oops: 0000 [#1] SMP 
Sep 24 06:52:35 fish kernel: [    7.951943] Modules linked in: i82975x_edac(+) snd edac_core serio_raw soundcore mac_hid hid_generic usbhid hid ahci firewire_ohci libahci firewire_core crc_itu_t floppy tg3 ptp pps_core
Sep 24 06:52:35 fish kernel: [    7.952012] CPU: 0 PID: 309 Comm: systemd-udevd Not tainted 3.13.0-36-generic #63-Ubuntu
Sep 24 06:52:35 fish kernel: [    7.952012] Hardware name: Dell Inc.                 Precision WorkStation 390    /0DN075, BIOS 2.6.0  05/19/2008
Sep 24 06:52:35 fish kernel: [    7.952012] task: ffff88007b988000 ti: ffff8800796e8000 task.ti: ffff8800796e8000
Sep 24 06:52:35 fish kernel: [    7.952012] RIP: 0010:[<ffffffffa00c97ee>]  [<ffffffffa00c97ee>] 

======= next line shows the culprit 
edac_create_sysfs_mci_device+0x23e/0x550 [edac_core]
Sep 24 06:52:35 fish kernel: [    7.952012] RSP: 0018:ffff8800796e9aa8  EFLAGS: 00010246
Sep 24 06:52:35 fish kernel: [    7.952012] RAX: ffffea0000d91300 RBX: ffff880078b09000 RCX: 0000000000017280
Sep 24 06:52:35 fish kernel: [    7.952012] RDX: 0000000000000000 RSI: 0000000000000002 RDI: 000000000000001f
Sep 24 06:52:35 fish kernel: [    7.952012] RBP: ffff8800796e9ae8 R08: 0000000000000000 R09: 0000000000000000
Sep 24 06:52:35 fish kernel: [    7.952012] R10: ffffea0001e58000 R11: ffffffff814947b4 R12: 00000000ffffffff
Sep 24 06:52:35 fish kernel: [    7.952012] R13: 00000000ffffffff R14: ffff880036b88000 R15: ffff880036b88000
Sep 24 06:52:35 fish kernel: [    7.952012] FS:  00007fd9c2c22880(0000) GS:ffff88007f800000(0000) knlGS:0000000000000000
Sep 24 06:52:35 fish kernel: [    7.952012] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Sep 24 06:52:35 fish kernel: [    7.952012] CR2: 0000000000000000 CR3: 00000000796d0000 CR4: 00000000000007f0
Sep 24 06:52:35 fish kernel: [    7.952012] Stack:
Sep 24 06:52:35 fish kernel: [    7.952012]  ffff8800796e9b60 ffff8800796e9b65 ffff8800ffffffed ffff880078b09400
Sep 24 06:52:35 fish kernel: [    7.952012]  ffff880036b88000 0000000000000008 ffff880036b88000 ffff880036b88000
Sep 24 06:52:35 fish kernel: [    7.952012]  ffff8800796e9b18 ffffffffa00c7547 ffff88003646dc00 ffff880078b09400
Sep 24 06:52:35 fish kernel: [    7.952012] Call Trace:
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffffa00c7547>] edac_mc_add_mc+0x157/0x370 [edac_core]
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffffa00d7556>] i82975x_init_one+0x376/0x410 [i82975x_edac]
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff813a84a5>] local_pci_probe+0x45/0xa0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff813a9745>] ? pci_match_device+0xc5/0xd0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff813a9869>] pci_device_probe+0xd9/0x130
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff8149596d>] driver_probe_device+0x12d/0x3e0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff81495cf3>] __driver_attach+0x93/0xa0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff81495c60>] ? __device_attach+0x40/0x40
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff814938d3>] bus_for_each_dev+0x63/0xa0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff8149531e>] driver_attach+0x1e/0x20
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff81494f00>] bus_add_driver+0x180/0x250
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffffa0006000>] ? 0xffffffffa0005fff
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff81496374>] driver_register+0x64/0xf0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffffa0006000>] ? 0xffffffffa0005fff
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff813a7e3c>] __pci_register_driver+0x4c/0x50
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffffa0006032>] i82975x_init+0x32/0x1000 [i82975x_edac]
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff8100214a>] do_one_initcall+0xfa/0x1b0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff810598d3>] ? set_memory_nx+0x43/0x50
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff810e21ed>] load_module+0x12dd/0x1b40
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff810ddc80>] ? store_uevent+0x40/0x40
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff810e2bc6>] SyS_finit_module+0x86/0xb0
Sep 24 06:52:35 fish kernel: [    7.952012]  [<ffffffff8172efad>] system_call_fastpath+0x1a/0x1f
Sep 24 06:52:35 fish kernel: [    7.952012] Code: 45 d0 75 d3 66 0f 1f 84 00 00 00 00 00 4c 89 ff 89 45 d0 e8 75 80 3c e1 49 8b bf a0 02 00 00 e8 29 af 3c e1 49 8b 97 a0 02 00 00 <48> 8b 3a e8 ca 7d 0d e1 8b 45 d0 48 83 c4 18 5b 41 5c 41 5d 41 
Sep 24 06:52:35 fish kernel: [    7.952012] RIP  [<ffffffffa00c97ee>] edac_create_sysfs_mci_device+0x23e/0x550 [edac_core]
Sep 24 06:52:35 fish kernel: [    7.952012]  RSP <ffff8800796e9aa8>
Sep 24 06:52:35 fish kernel: [    7.952012] CR2: 0000000000000000
Sep 24 06:52:35 fish kernel: [    7.959078] ---[ end trace d79a242bd61151eb ]---


After rebooting (thanks to MagicSysReq)  the machine shuts down properly.
Shutdown does not display message 'mount  / busy' anymore.

I don't know what consequences I will encounter by not having loaded the i82975x_edac module.
As far as I can say, Trusty Tahr and I are quite happy without it.

ahau