systemd unit file should use owserver --foreground

[kalvdans]

After upgrading from Ubuntu 18.04 to 20.04 I started to see errors when restarting owserver.service. They appear approximately every second time without clues from the logs.
```
$ sudo systemctl stop owserver.service; sudo systemctl start owserver.service
Job for owserver.service failed because the service did not take the steps required by its unit configuration.
See "systemctl status owserver.service" and "journalctl -xe" for details.

$ systemctl status owserver.service
● owserver.service - Backend server for 1-wire control
     Loaded: loaded (/lib/systemd/system/owserver.service; disabled; vendor preset: enabled)
     Active: active (running) since Wed 2020-09-30 12:40:53 UTC; 7s ago
       Docs: man:owserver(1)
   Main PID: 13861 (owserver)
      Tasks: 2 (limit: 18965)
     Memory: 1.5M
     CGroup: /system.slice/owserver.service
             └─13861 /usr/bin/owserver -c /etc/owfs.conf

Sep 30 12:40:53 arvid-m12 systemd[1]: Starting Backend server for 1-wire control...
Sep 30 12:40:53 arvid-m12 OWFS[13861]: DEFAULT: ow_daemon.c:(144) Entered background mode, quitting.
Sep 30 12:40:53 arvid-m12 OWFS[13861]: DEFAULT: ow_usb_msg.c:(214) Opened USB DS9490 bus master at 1:3.
Sep 30 12:40:53 arvid-m12 OWFS[13861]: DEFAULT: ow_usb_cycle.c:(128) Set DS9490 1:3 unique id to 81 2C B9 3C 00 00 00 87
Sep 30 12:40:53 arvid-m12 systemd[1]: Started Backend server for 1-wire control.
```

Versions:
```
$ dpkg --list | grep " ow"
ii owfs-common 3.2p3+dfsg1-5build1 all common files used by any of the OWFS programs
ii owserver 3.2p3+dfsg1-5build1 amd64 Backend server for 1-Wire control
$ systemd --version
systemd 245 (245.4-4ubuntu3.2)
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=hybrid
```

According to https://github.com/owfs/owfs/issues/77#issuecomment-701480249 the solution is to add --foreground flag to owserver.service file, and it seems to work.