mp_common: improve OSD/status time formatting

Allow negative times. Timestamps can be negative, and we actually
display negative time for other reasons too, such as when waiting for
the old audio to drain with gapless audio.)

Avoid overflows with relatively large time values. (We still don't
handle values too large for int64_t.)
demux_lavf: remove "internet radio hack"

It appears this is not needed anymore. ffmpeg can handle "chained" ogg
files fine. These can be created with "cat file1.ogg file2.ogg > chained.ogg",
and are similar (or equal) to some internet radio streams. Apparently
ffmpeg used to add new tracks when crossing boundaries in chained files,
and the hack in demux_lavf.c handled this. At some later point, ffmpeg's
ogg demuxer was improved, and stopped adding new tracks as long as the
codec doesn't change.

Since the hack in demux_lavf.c was hardcoded to Vorbis (i.e. only active
if the new and old track were both Vorbis), it's dead code, and we can
remove it. I couldn't find any stream that triggered this hack, or fails
without it.

Firefox had a similar issue, and its bug tracker makes a good reference:

    https://bugzilla.mozilla.org/show_bug.cgi?id=455165

NOTE: this doesn't update metadata on track changes anymore.
demux_lavf: remove weird stream auto-selection

Should be dead code. Stream selection is handled either during
demuxer initialization, or via DEMUXER_CTRL_SWITCH_*.
(If there were actually situations where this code did something, it
was probably broken anyway.)
mplayer: make advancing the playlist respect looping

Explicitly advancing the playlist with input commands ("playlist_next")
didn't jump back to the first file, if the current file was the last on
the playlist and looping was enabled.

Fix this and make the behavior with explicit input and playback EOF the
same.

Also add a minor feature: if looping is enabled, and the current file is
the first on the playlist, going back one entry jumps to the last
playlist entry (without changing loop count).

Fixes #22.
Copyright: cleanup entries for removed code
DOCS: remove files documenting removed/rewritten functionality

Most of these are a waste of time. Some (like slave.txt) have been
rewritten in rst.

The remaining files aren't that useful, but probably do no harm.
vf_dlopen: fix external pixel format names

"yv12" stopped working a while ago, and so did other FourCC-like names.
Make "yv12" work again, so that the examples in TOOLS/vf_dlopen/ work.
demux_lavf: default PTS to 0 for attached pictures

The ffmpeg/libav attached picture hack usually set the PTS of video
packets to AV_NOPTS_VALUE. Set it to 0 to avoid printing a warning by
the filter code.
manpage: document option syntax variations better