journal: Don't use loginuid if it's not valid

Code above this attempted to load loginuid, if this failed for
whatever reason, we'd still end up using that value (0) in place of
realuid.  Fix this by setting a bool when we know the loginuid is
valid.

This fixes journal messages showing up in per-user journals in
gnome-ostree (not configured with loginuid, but I'll shortly fix
that).