Make audio_driver_lookup() try load the module in case it doesn't find
the driver in the registry.  Also load all modules for -audio-help, so
the help output includes the help text for modular audio drivers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180306074053.22856-3-kraxel@redhat.com