cookies: don't read cookie files from ancient browsers

Remove the code that attempted to read cookie files from well-known
browser locations. This code was written for ancient browsers, and only
knew about Mozilla and Netscape. While it's possible that these browsers
are still alive and still use the same config locations and cookie file
formats, the only Mozilla-based browser that still matters is Firefox.
Firefox uses a sqlite database for cookies, located in a slightly
different config path.

Just remove this code.
options: rename -ss and -endpos, allow relative times

Rename the -ss option to -start, and -endpos to -length. Add a -end
option. The -end option always specifies an absolute end time, as
opposed to -endpos/-length.

All these options (--start, --end, --length) now accept relative times.
Percent positions (e.g. "--start=30%") are interpreted as fractions of
the file duration. Negative times (e.g. "--start=-1:00) are interpreted
relative to the end of the file. Chapters (e.g. "--start=#3") yield the
chapter's time position.

The chapter support might be able to replace --chapter completely, but
for now I am not sure how well this works out with e.g. DVDs and BDs,
and a separate --chapter option is useful interface-wise.
options: --field-dominance: replace magic number values with choices

--field-dominance=-1  becomes --field-dominance=auto
--field-dominance=0   becomes --field-dominance=top
--field-dominance=1   becomes --field-dominance=bottom
options: --cursor-autohide-delay: replace magic number values

--cursor-autohide-delay=-2 becomes --cursor-autohide-delay=always
--cursor-autohide-delay=-1 becomes --cursor-autohide-delay=no
options: rename -sub-fuzziness to -autosub-match, change option values

"--autosub-match" is close to "--autosub", and reflects what this
option does slightly better. Replace the magic number option values
with choices:

--sub-fuzziness=0  becomes --autosub-match=exact
--sub-fuzziness=1  becomes --autosub-match=fuzzy
--sub-fuzziness=2  becomes --autosub-match=all
options: add --mute for setting initial audio mute status

Similar to --volume. Takes this as opportunity to move the variable
corresponding to --volume into MPOpts.
subreader: replace sub_free() by talloc destructor

Makes it less annoying to free the sub_data.
command: add sub_reload and sub_remove commands

sub_remove remove an external subtitle track, for whatever this may be
needed.

sub_reload removes and re-adds an external subtitle track.

Also rename sub_load to sub_add, because that seems to be more in line
with sub_remove.
core: add --keep-open, which doesn't close the file on EOF

The --keep-open option causes mpv not to close the current file.
Instead, it will pause, and allow the user to seek around. When
seeking beyond the end of the file, mpv does a precise seek back to
the previous last known position that produced video output.

In some corner cases, mpv might not be able to produce video output at
all, despite having created a VO. (Possibly when only 1 frame could be
decoded, but the video filter chain queues frames. Then a VO would be
created, without sending an actual video frame to the VO.) In these
cases, the VO window will not redraw, not even OSD.

Based on a patch by coax [1].

[1] http://devel.mplayer2.org/ticket/210#comment:4
mplayer: refactor: move computation of a/v sync

The computation for the A/V sync value was inside print_status(). Move
it into its own function; this makes things simpler and gets rid of some
minor dead code.
subtitles: improve support for libavformat demuxed subtitles

Make demux_lavf not error out if no video or audio track is present.
This allows opening subtitle files with the demuxer.

Improve the test whether subtitles read from demuxers must do explicit
packet reads. (I'm not sure whether always doing these reads could have
bad effects, such as reading too many audio and video packets at once,
so be conservative.)