demux_mkv: fix undefined behavior

With some files, the extradata variable can remain uninitialized, but
will be used for memory access.

CC: @mpv-player/stable (with high priority)
player: signal EOF when using --frames
player: fix --frames

This could produce an extra frame, because reaching the maximum merely
signals the playloop to exit, without strictly enforcing the limit.

Fixes #1181.

CC: @mpv-player/stable
player: fix crash on early audio uninit

Could crash when exiting playback in very early stages of
initialization.

CC: @mpv-player/stable
manpage: fix reference to a defunct option

The change was made with faad40aad92d51290ef2fc4d94277eca89863873.
common: fix \x-style escaping

This was rejecting correct escapes and accepting incorrect ones.
player: quote %-starting strings

Leading percent sign is a quote indicator so it needs to be quoted
itself.
terminal: recognize ^h

Fixes #1185.

CC: @mpv-player/stable
audio: don't go to sleep after audio reinit

It possibly goes to sleep without actually starting to decode audio.
Possibly fixes a problem with --no-osc --no-video reported on IRC.

CC: @mpv-player/stable
sub: adjustments to --ass-style-override option

Now requires newest libass git. Since this feature wasn't part of a
libass release yet, I'm not bothering making the mpv code compatible
with as how it was previously implemented (it will just be disabled
with any older libass).

CC: @mpv-player/stable (because mpv-build uses libass git, and this
                        breaks the feature)
player: fix exiting if both audio and video fail initializing

The player was supposed to exit playback if both video and audio failed
to initialize (or if one of the streams was not selected when the other
stream failed). This didn't work; for one this check was missing from
one of the failure paths. And more importantly, both checked the
current_track array incorrectly.

Fix these issues, and move the failure handling code into a common
function.

CC: @mpv-player/stable

Conflicts:
	player/audio.c
	player/video.c
Release 0.6.2