journalctl: add with-unit mode

journalctl: add with-unit mode

When dealing with a large number of template instances, for example
when launching daemons per VRF, it is hard for operators to correlate
log lines to arguments.
Add a new with-unit mode which, if available, prefixes unit and user
unit names when displaying its log messages instead of the syslog
identifier. It will also use the full timestamp with timezones, like
the short-full mode.