sndio: Add module.

- can control sndio: change volume, toggle mute
- appearance is somewhat dynamic: takes muted status into account
- uses polling inside sleeper thread to update values
- uses sioctl_* functions, requires sndio>=1.7.0.

include/factory.hpp

@@ -39,6 +39,9 @@
 #ifdef HAVE_LIBMPDCLIENT
 #include "modules/mpd.hpp"
 #endif
+#ifdef HAVE_LIBSNDIO
+#include "modules/sndio.hpp"
+#endif
 #include "bar.hpp"
 #include "modules/custom.hpp"
 #include "modules/temperature.hpp"