core: fix output (logging) for mount units (#7603)

core: fix output (logging) for mount units (#7603)

Documentation - systemd.exec - strongly implies mount units get logging.

It is safe for mounts to depend on systemd-journald.socket.  There is no
cyclic dependency generated.  This is because the root, -.mount, was
already deliberately set to EXEC_OUTPUT_NULL.  See comment in
mount_load_root_mount().  And /run is excluded from being a mount unit.

Nor does systemd-journald depend on /var.  It starts earlier, initially
logging to /run.

Tested before/after using `systemctl stop tmp.mount`.