logind-user: take gc_mode into account when reporting user state

logind-user: take gc_mode into account when reporting user state

Prompted by #33650

Previously, if a user manually starts user@.service (which is
something we support), we'd track it as 'manager' session.
However, since user_get_state() ignores all non-pinning sessions,
if lingering is not enabled, the user state would always be
reported as 'closing', which is spurious.

Let's instead take gc_mode into consideration, and ignore
non-pinning sessions only if USER_GC_BY_PIN.