dec_sub: don't print sub charset of it's empty
dec_sub: fix memory leak when using subtitle codepage conversion
options: remove -lavdopts debug suboption

This can be set as avopt instead.
options: rename -lavdopts to -vd-lavc, -lavfdopts to -demuxer-lavf

Also change manpage so that top-level options are documented instead
of suboptions. Suboptions still work, but might go away eventually.
options: rename --rawvideo to --demuxer-rawvideo, same with --rawaudio
options: rename --mkv-subtitle-preroll, --dtshd

We still keep the old names as alias for short-time compatibility.
stream: redo memory streams

Make memory streams actual streams. This causes fewer weird corner cases
and actually allows using demuxers with them.
core: rename mplayer.h and quvi.c

mplayer.h used to be used for much more stuff, but all what is left are
quvi related definitions. Rename quvi.c as well to make its purpose
clearer.
core: add libquvi 0.9 support

This adds support for libquvi 0.9.x, and these features:
- start time (part of youtube URL)
- youtube subtitles
- alternative source switching ('l' and 'L' keys)
- youtube playlists

Note that libquvi 0.9 is still in development. Although this seems to
be API stable now, it looks like there will be a 1.0 release, which is
supposed to be the next stable release and the actual successor of
libquvi 0.4.x.
configure: prefer libquvi 0.4.x over libquvi 0.9.x

Because 0.4.x is the current series of stable releases.