journal: log _SYSTEMD_USER_UNIT for user session units
core: log USER_UNIT instead of UNIT if in user session
logs-show: add show_journal_by_user_unit

Print the journal for a user session unit. For now this filters by
_SYSTEMD_USER_UNIT and USER_UNIT and additionally _UID.
systemctl: print the user session journal for user session units.
man: document the _SYSTEMD_USER_UNIT journal field
core/group-util: merge two functions
core/cgroup-util: simplify functions and add tests