configure: cocoa: link to libarclite

libarclite provides method stubs for the Subscripting headers added in
0407869ae3. This allows to correclty build mpv on OSX 10.7 (I had tested that
commit with OSX 10.8 running 10.7 SDK).

It seems on 10.8 this option does't make any difference in the linked libraries
(checked with otool -L) so I just add it unconditionally.

Warning: This doesn't mean mpv moved to ARC. To do that one would have to add
`-fobjc-arc` to the cflags.
av_log: change how ffmpeg log messages are formatted

Don't print the ffmpeg context pointer as %p. This is usually useless
and confusing. Prefix all messages with "ffmpeg" to make clear that
ffmpeg is printing these messages, and not us.

If libavcodec is from Libav, use "libav" as prefix instead. (In theory,
FFmpeg/Libav libraries could be mixed, but I don't think that is
actually possible in practice.)
cache: cache number of chapters

Querying this caused the cache to block and wait. Some parts of the
frontend (like progress bar) call this very often, so cache performance
was ruined in these cases.

Also print a message in -v mode when the cache is blocked for a
STREAM_CTRL. This should make debugging similar issues easier.
Merge remote-tracking branch 'origin/master' into lua_experiment