debathena-inhibit-dbus-helper file handled badly

[jdreed]

debathena-reactivate's dbus-daemon-launch-helper makes use of this file to ensure dbus daemons are spawned outside the chroot when the file is present. The file is created in the schroot setup.d hook. Unfortunately, if a chroot session is left lying around upon reboot, the file is _created_ when that session is force-ended by the schroot init.d script and is not cleaned up, leaving the file present for the next reactivate session. We try to remove it in snapshot-run to avoid this, but as it's owned by root, that never works. We should fix this.