core: apply LogLevelMax to messages about units

core: apply LogLevelMax to messages about units

This commit applies the filtering imposed by LogLevelMax on a unit's
processes to messages logged by PID1 about the unit as well.

The target use case for this feature is a service that runs on a timer
many times an hour, where the system administrator decides that writing
a generic success message to the journal every few minutes or seconds
adds no diagnostic value and isn't worth the clutter or disk I/O.