ao: remove redundant get_delay check

It did nothing; the real check is in push.c.
stream_dvd: fix seeking regression

This was accidentally completely destroyed with commit 24f1878e. I
didn't notice it when testing, because forward seeking still worked
mostly.

The issue was that dvd_seek_to_time() actually called stream_seek(),
which was supposed to call the byte-level seek function dvd_seek(). So
we have to restore this function, and replace all generic stream calls
with stream_dvd.c internal ones. This also affects stream->pos (now a
random number as far as stream_dvd.c is concerned) and stream_skip().
stream_dvdnav: print more debugging info
msg: correct ringbuffer log level comparison