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.

src/libsystemd/sd-login/sd-login.c

@@ -687,7 +687,7 @@ _public_ int sd_seat_get_sessions(const char *seat, char ***sessions, uid_t **ui
 
         r = parse_env_file(p, NEWLINE,
                            "SESSIONS", &s,
-                           "ACTIVE_SESSIONS", &t,
+                           "UIDS", &t,
                            NULL);
         if (r == -ENOENT)
                 return -ENXIO;