sd-login: read list of uids of sessions from UIDS not ACTIVE_SESSIONS
As described by Luke Shumaker:
sd_seat_get_sessions looks at /run/systemd/seats/${seat_name}:SESSIONS to get
the list of sessions (which I believe is correct), and at
/run/systemd/seats/${seat_name}:ACTIVE_SESSIONS for the list of users (which
I believe is incorrect); I believe that it should look at the UIDS field for
the list of users. As far as I can tell, the ACTIVE_SESSIONS field is never
even present in the seats file. I also believe that this has been broken
since the function was first committed almost 6 years ago.
Fixes #5743.