cache: fix checks/output on initialization

resize_cache() checks the size itself and clamps the size to the valid
range if necessary, so we don't need these checks. In fact, the checks
are different. Also, output the cache size after clamping, instead of
before.
cache: change a define to an enum

More consistent.
cache: fix description of the offset field

This field sure is a bit strange. I hope the description is correct now.
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.