stream_dvd, cache: hack seeking with --cache + dvd:// back into working

This was broken at some unknown point (even before the recent cache
changes). There are several problems:
- stream_dvd returning a random stream position, confusing the cache
  layer (cached data and stream data lost their 1:1 corrospondence by
  position)
- this also confused the mechanism added with commit a9671524, which
  basically triggered random seeking (although this was not the only
  problem)
- demux_lavf requesting seeks in the stream layer, which resulted in
  seeks in the cache or the real stream

Fix this by completely removing byte-based seeking from stream_dvd. This
already works fine for stream_dvdnav and stream_bluray. Now all these
streams do time-based seeks, and pretend to be infinite streams of data,
and the rest of the player simply doesn't care about the stream byte
positions.

stream/cache.c

@@ -233,7 +233,7 @@ static bool cache_fill(struct priv *s)
         cache_drop_contents(s);
     }
 
-    if (stream_tell(s->stream) != s->max_filepos) {
+    if (stream_tell(s->stream) != s->max_filepos && s->seekable) {
         MP_VERBOSE(s, "Seeking underlying stream: %"PRId64" -> %"PRId64"\n",
                    stream_tell(s->stream), s->max_filepos);
         stream_seek(s->stream, s->max_filepos);