ao_write: Avoid pthread_join().

The call to pthread_join() may block for hundreds of milliseconds,
depending of the amount of buffered data. This is not acceptable in
a post_select hook. Among other problems this leads to an incorrect time
display and fake "time jump" warnings.

This patch drops the call to pthread_join() and makes ->post_select()
return non-negative until the play thread has terminated. In order to
have a way to tell if the thread is still running we let the thread
(i.e. aow_play()) remove its own buffer tree node on exit instead
of doing this in ->post_select() after pthread_join(). Hence if
->thread_btrn is NULL, we know the thread has terminated. It would
be easier to use pthread_tryjoin_np() but this would not be portable,
as this function is a non-standard GNU extension.