osc, dvd, bd: fix mouse state when changing menu modes

The flags weren't correctly set, and the mouse cursor remained visible
after leaving menu mode.

This was apparently broken in 0.7.0 too.

Fixes #1316.
ao_alsa: hackfix mono playback

ALSA returns "FL" as channel layout when trying to play mono. mpv and
libavresample don't like this; in particular, using libavresample to
convert stereo to "FL" fails.
ao_alsa: minor simplification

Whether we print it as warning or error doesn't really matter; we
continue anyway. (I don't actually know what the implications of running
in non-blocking mode are; for what's it worth, when I tested with
explicitly changing to non-blocking, it seemed to work fine anyway, so
don't change that part.)