Sessions monitor should watch only changes in sessions

Currently, the polkitbackendsessionmonitor watches all notifications from
sd-login-monitor. This causes waking polkit even four+ times on every
login/logout and starting a train of action re-verification several
times for each instantiated PolkitAuthority (e.g. applets on gnome-shell
panel that utilize PolkitPermission).
This is not necessary, because polkit only needs to watch for sessions
that change status from active to online and vice-versa (to maintain
security for actions that have is_active defined).
This still doesn't resolve the problem completely, but at least halves
the impact on system resources.

src/polkitbackend/polkitbackendsessionmonitor-systemd.c

@@ -106,7 +106,7 @@ sd_source_new (void)
   source = g_source_new (&sd_source_funcs, sizeof (SdSource));
   sd_source = (SdSource *)source;
 
-  if ((ret = sd_login_monitor_new (NULL, &sd_source->monitor)) < 0)
+  if ((ret = sd_login_monitor_new ("session", &sd_source->monitor)) < 0)
     {
       g_printerr ("Error getting login monitor: %d", ret);
     }