This extends the "uid:gid" syntax for "u" lines so that a group
name can be given instead of a GID. This requires that the group
is either queued for creation by sysusers, or it is already defined
on the system.
Closes#14340
The unit load queue can be processed in the middle of setting the
unit's properties, so its load_state would no longer be UNIT_STUB
for the check in bus_unit_set_properties(), which would cause it to
incorrectly return an error.
David Michael