demux_lavf: add workaround for broken libavformat seek behavior

Seeking before the start of a .flac file (such as seeking backwards when
the file just started) generates a bunch of decoding errors and audible
artifacts. Also, the audio output doesn't match the reported playback
position. The errors printed to the terminal are:

[flac @ 0x8aca1c0]invalid sync code
[flac @ 0x8aca1c0]invalid frame header
[flac @ 0x8aca1c0]decode_frame() failed

This is most likely a problem with the libavformat API. When seeking
with av_seek_frame() fails, the demuxer can be left in an inconsistent
state. ffplay has the same issue [1].

Older versions of mpv somehow handled this fine. Bisection shows that
commit b3fb7c2 caused this regression by removing code that retried
failed seeks with an inverted AVSEEK_FLAG_BACKWARD flag. This code was
removed because it made it harder to stop playback of a file by seeking
past the end of the file (expecting this is rather natural when skipping
through multiple files by seeking, and the internal mplayer demuxers
also did this).

As a workaround, re-add the original code, but only for the backwards
seeking case.

Also note that the original intention of the code removed in b3fb7c2 was
not dealing with this case, but something else. It also had to do with
working around weird libavformat situations, though. It's not perfectly
clear what exactly. See commit 1ad332f.

[1] https://ffmpeg.org/trac/ffmpeg/ticket/2282
cocoa_common: fix crash with dead key input and simplify related code

Fixes #29. When a user used dead input keys (like the accent key), `mpv`
crashed because the code tried to access the 0 element of a characters array
(which was empty).

While I was closing this bug, I refactored some related conditionals to
make the code more readable.
mplayer: don't display "-1" as chapter when chapter seek fails

Increase robustness against out of bound chapter numbers. Normally
these functions expect that the callers sanitize the chapter number.
This went wrong at least in add_seek_osd_messages() (which displayed
a chapter "-1" when chapters were not available). Make these functions
a bit friendler and add some reasonable checks and fallbacks, which
fixes the mentioned chapter seeking case as well.
osd: prevent osd bar from sticking around on seeks

This was supposed to be fixed in f897138, but there's another corner
case. Basically, set_osd_function() reset the OSD time, which is not
nice at all and breaks the logic of letting OSD elements disappear when
they're not wanted anymore. Fix this by adding a separate timer for
this.

Additionally, make sure the OSD bar is _really_ always updated when
visible. Also, redraw the OSD only if the OSD bar actually changes to
prevent redrawing too often (every vo_osd_changed() will flag that the
OSD should be redrawn, even if nothing changes).
options: parse C-style escapes for some options

Being able to insert newline characters ("\n") is useful for
--osd-status-msg, and possibly also for anything that prints to the
terminal. Espcially --term-osd-esc looks relatively useless without
being able to specify escapes.

Maybe parsing escapes should happen during command line / config parsing
instead (for all options).
vd_lavc: fix software decoding fallback

The string is deallocated by the callee after initialization, so
fallback at runtime passes a deallocated string to libavcodec, which
results in random crashes. Regression introduced by commit 4d016a9.
x11_common: uncrustify

Also some other cosmetic changes. And reformat the two remaining doxygen
comments.

Removing MSGLEN in x11_errorhandler() is technically not just a cosmetic
change, but the result is the same anyway.
core: move `xineramascreen` to `MPOpts` as `vo_screen_id`

This is a small cleanup in preparation for the next commit.
core: add fs-screen option for fullscreen display selection

`--fs-screen` allows to decide what display to go fullscreen into. The
semantics of `--screen` changed and now it is only used to select the windowed
display when starting the application.

This is useful for people using mpv with an external TV. They will start
windowed on their laptop's screen and switch to fullscreen on the TV.

@wm4 worked on the x11 and w32 parts of the code. All is squashed in one
commit for history clarity.
Merge remote-tracking branch 'mpv-player/master'
wayland: implement --fs-screen option

This replaces the --screen option internal and ignores it for now, because
wayland clients can only choose the output for going fullscreen but can't
choose an specific output for a floating window.

Conflicts:
	video/out/wayland_common.c