expose subcommand in log messages

[Karl-Philipp Richter]

`dpkg` logs messages of all subcommands without any distinction. In the following excerpt of `dpkg` output

    update-initramfs: Generating /boot/initrd.img-4.0.0-4-generic
    df: Warnung: Lesen der Tabelle eingehängter Dateisysteme nicht möglich: Datei oder Verzeichnis nicht gefunden
    run-parts: executing /etc/kernel/postinst.d/pm-utils 4.0.0-4-generic /boot/vmlinuz-4.0.0-4-generic
    run-parts: executing /etc/kernel/postinst.d/unattended-upgrades 4.0.0-4-generic /boot/vmlinuz-4.0.0-4-generic
    run-parts: executing /etc/kernel/postinst.d/update-notifier 4.0.0-4-generic /boot/vmlinuz-4.0.0-4-generic
    run-parts: executing /etc/kernel/postinst.d/zz-update-grub 4.0.0-4-generic /boot/vmlinuz-4.0.0-4-generic
    Grub-Konfigurationsdatei wird generiert …
    Warnung: Werte ungleich 0 für »GRUB_TIMEOUT« werden, falls »GRUB_HIDDEN_TIMEOUT« aktiviert ist, nicht mehr unterstützt.
    Linux-Abbild gefunden: /boot/vmlinuz-4.1.1-040101-generic
    initrd-Abbild gefunden: /boot/initrd.img-4.1.1-040101-generic
    Linux-Abbild gefunden: /boot/vmlinuz-4.0.0-4-generic
    initrd-Abbild gefunden: /boot/initrd.img-4.0.0-4-generic
    Ubuntu 12.04.5 LTS (12.04) auf /dev/sdb6 gefunden
    Adding boot menu entry for EFI firmware configuration
    erledigt
    linux-libc-dev:amd64 (4.0.0-4.7) wird eingerichtet ...
    policykit-1 (0.105-11) wird eingerichtet ...
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    network-manager (0.9.10.0-4ubuntu18) wird eingerichtet ...
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    Running in chroot, ignoring request.
    python-qt4 (4.11.4+dfsg-1) wird eingerichtet ...

some subcommands (e.g. `update-initramfs`) are exposed, others not. The user can not know e.g. which command invokes `df` (log message should be `[command]: df: Warnung...`). Some things can be guessed (e.g. that `grub` causes `Adding boot menu entry for EFI firmware configuration`) other are impossible to figure out, e.g. which command is skipped because of `dpkg` running in `chroot`.

Prefixing all log entries with `[command]: ` would help a lot.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: dpkg 1.17.25ubuntu1
Uname: Linux 4.1.1-040101-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Jul 15 14:30:41 2015
InstallationDate: Installed on 2015-06-08 (37 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: dpkg
UpgradeStatus: Upgraded to vivid on 2015-06-09 (36 days ago)