Add dpkg trigger for /usr/lib/systemd/catalog to update catalog db.

Add a call to journalctl --update-catalog to postinst to update
the Message Catalog database.

Add a systemd.postrm to remove /var/lib/systemd/catalog on purge.

This compiles the files from /usr/lib/systemd/catalog into a
binary database, which is used by journalctl to provide
additional information for log entries.

Also see http://www.freedesktop.org/wiki/Software/systemd/catalog/