ao_oss: improve format negotiation, and hopefully fix pass-through

Digital pass-through was probably broken. Possibly fix it (no way to
test). This also should make the logic slightly saner.

Fortunately, it's unlikely that anyone who uses OSS has a spdif setup.
player: --loop-file takes precedence before --keep-open
player: show correct playback time with --keep-open --no-video

Whatever.
player: change --keep-open semantics

By popular request.