units: order systemd-user-sessions.service after network.target

That way we can be sure that local users are logged out before the network is
shut down when the system goes down, so that SSH session should be ending
cleanly before the system goes down.

Fixes: #2390

units/systemd-user-sessions.service.in

@@ -8,7 +8,7 @@
 [Unit]
 Description=Permit User Sessions
 Documentation=man:systemd-user-sessions.service(8)
-After=remote-fs.target nss-user-lookup.target
+After=remote-fs.target nss-user-lookup.target network.target
 
 [Service]
 Type=oneshot