launcher: add logind backend

Instead of connecting to weston-launch from launcher-util, we now try to
connect to logind first. If logind provides session-devices, we use them.
If not, we fall back to the old weston-launch facility.
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index 4454599..cbdbb3c 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -2623,7 +2623,8 @@
 	}
 
 	/* Check if we run drm-backend using weston-launch */
-	ec->base.launcher = weston_launcher_connect(&ec->base, param->tty);
+	ec->base.launcher = weston_launcher_connect(&ec->base, param->tty,
+						    param->seat_id);
 	if (ec->base.launcher == NULL) {
 		weston_log("fatal: drm backend should be run "
 			   "using weston-launch binary or as root\n");