Only use the user unit for pane scope dependencies

Pane scopes are created in the user manager, but when the server is not
in a user unit, for example when it was started from a login session's
session-N.scope, sd_pid_get_unit() returned that system unit and it was
set as Before and PartOf of the pane scope. The user manager cannot see
system units, so this only leaves a reference to a missing unit.

Set the dependencies only when sd_pid_get_user_unit() finds a unit.
GitHub issue 5741.

compat/systemd.c

@@ -234,8 +234,7 @@ systemd_new_scope_message(sd_bus *bus, int oom_policy, sd_bus_message **mp,
 	 * as dependent on it. Use "Before" to make sure systemd will not try to
 	 * kill them first.
 	 */
-	if (sd_pid_get_user_unit(parent_pid, &unit) == 0 ||
-	    sd_pid_get_unit(parent_pid, &unit) == 0) {
+	if (sd_pid_get_user_unit(parent_pid, &unit) == 0) {
 		r = sd_bus_message_append(*mp, "(sv)", "Before", "as", 1,
 		    unit);
 		if (r >= 0) {