player: readd code accidentally removed with commit 61efe87e

Oops.
audio: use symbolic constants instead of magic integers

Similar to commit 26468743.
player: simplify logic on video errors

Fatal errors in the vidoe chain (such as failing to initialize the video
chain) disable video decoding. Restart the playloop, instead of just
continuing the current iteration.

The resulting behavior should be the same, but it gets rid of possible
corner cases.
manpage: fix wording for --aid
player: simplify a condition

Move a condition somewhere else, which makes it conceptually simpler.

Also, the assignment to full_audio_buffers removed with this commit was
dead, and its value never used.