Make usage of pidof optional in maintscripts

Michael Bieblrequested to merge
pidof-optional into debian/unstable

The usage of pidof in dbus.postinst is not strictly required. So make it optional to allow sysvinit-utils to become non-essential. Also drop the check for /bin/dbus-daemon while at it. The latest Ubuntu (LTS) releases of dbus use /usr/bin/dbus-daemon.

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