compositor: print usage to stdout on success (not stderr)

Triggered by weston --help, the usage() output should not look like an error.

Note that there is only one caller of usage() at the moment, but let's handle
this here based on the status in case we add other cases.

https://gitlab.freedesktop.org/wayland/weston/issues/112

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

compositor/main.c

@@ -438,7 +438,7 @@ verify_xdg_runtime_dir(void)
 static int
 usage(int error_code)
 {
-	fprintf(stderr,
+	fprintf(error_code == EXIT_SUCCESS ? stdout : stderr,
 		"Usage: weston [OPTIONS]\n\n"
 		"This is weston version " VERSION ", the Wayland reference compositor.\n"
 		"Weston supports multiple backends, and depending on which backend is in use\n"