journald: when logging about dropped messages, include more meta data

journald: when logging about dropped messages, include more meta data

When we drop messages of a unit, we log about. Let's add some structured
data to that. Let's include how many messages we dropped, but more
importantly, let's link up the message we generate to the unit we
dropped the messages from by using the "OBJECT" logic, i.e. by
generating OBJECT_SYSTEMD_UNIT= fields and suchlike, that "journalctl
-u" and friends already look for.

Fixes: #6494