ao_oss: use poll(), drop --disable-audio-select support

Replace select() usage with poll() (and reduce code duplication).

Also, while we're at it, drop --disable-audio-select, since it has the
wrong name anyway. And I have doubts that this is needed anywhere. If
it is, it should probably fallback to doing the right thing by default,
instead of requiring the user to do it manually. Since nobody has done
that yet, and since this configure option has been part of MPlayer ever
since ao_oss was added, it's probably safe to say it's not needed.

The '#ifdef SNDCTL_DSP_GETOSPACE' was pointless, since it's already used
unconditionally in another place.
ao_oss: fix audio device leak on error

Close the audio device if it was already opened, but the rest of
initialization failed.