Introduce weston-launch

weston-launch starts weston and provides mechanism
for weston to set/drop drm master, open a tty,
and read input devices without being root.

Execution is allowed for local-active sessions
or users in the group weston-launch.
diff --git a/src/compositor.c b/src/compositor.c
index 1fce69a..5c5331e 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2331,6 +2331,7 @@
 	const char *extensions;
 
 	ec->wl_display = display;
+	ec->launcher_sock = weston_environment_get_fd("WESTON_LAUNCHER_SOCK");
 
 	if (!wl_display_add_global(display, &wl_compositor_interface,
 				   ec, compositor_bind))