logs-show: export logic to add matches for units

After that functions which add matches, show_journal_by_unit
and show_journal_by_user_unit, become nearly identical, so
I merged them into one function.
journalctl: show systemd messages about unit for -u

journalctl -u unit is not very useful, because it doesn't show
systemd messages about starting, stopping, coredumps, etc,
like systemctl status unit does. Make it show the same
information using the same rules.

If somebody really want to see just messages from by the unit,
it is easy enough to use _SYSTEMD_UNIT=...
initctl: catch write error, use _cleanup_

!= operator always returns something nonnegative, so the
error condition was not caught.