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>