Add handling of Message Catalog files.

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. journalctl only uses
the binary database in /var/lib/systemd/catalog/database.

Call journalctl --update-catalog in systemd.postinst and when
triggered by dpkg to update the Message Catalog database.
Also add systemd.postrm to remove the database on purge.

See http://www.freedesktop.org/wiki/Software/systemd/catalog/