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-fbdev.c b/src/compositor-fbdev.c
index 4376345..80e708d 100644
--- a/src/compositor-fbdev.c
+++ b/src/compositor-fbdev.c
@@ -894,7 +894,7 @@
 	wl_signal_add(&compositor->base.session_signal,
 		      &compositor->session_listener);
 	compositor->base.launcher =
-		weston_launcher_connect(&compositor->base, param->tty);
+		weston_launcher_connect(&compositor->base, param->tty, "seat0");
 	if (!compositor->base.launcher) {
 		weston_log("fatal: fbdev backend should be run "
 			   "using weston-launch binary or as root\n");