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/elogind/seats/${seat_name}:SESSIONS to get
  the list of sessions (which I believe is correct), and at
  /run/elogind/seats/${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.