Remove the last remains of slave mode

Almost nothing was left of it.

The only thing this commit actually removes is support for reading
input commands from stdin. But you can emulate this via:

 --input-file=/dev/stdin --input-terminal=no

However, this won't work on Windows. Just use a named pipe.
win32: fix assertion failure

If OpenGL 3.x doesn't work, the fallback to legacy GL will call the
function to create the DC again, and it will assert. Just make it not
to, and also simplify the code a bit.

Fixes #974.
manpage: remove duplicated misplaced description of a command
command: add a property that returns a list of all properties

Also remove the undocumented Lua mp.property_list() function.
TOOLS: add test script for property change notifications
client API: minor optimizations for property notification

Internally, there are two mechanisms which can trigger property
notification as used with "observed" properties in the client API.

The first mechanism associates events with a group of properties that
are potentially changed by a certain event. mp_event_property_change[]
declares these associations, and maps each event to a set of strings.
When an event happens, the set of strings is matched against the list of
observed properties of each client. Make this more efficient by
comparing bitsets of events instead. This way, only a bit-wise "and" is
needed for each observed property. Even better, we can completely skip
clients which have no observed properties that match.

The second mechanism just updates individual properties explicitly by
name. Optimize this by using the property index instead. It would be
nice if we could reuse the first mechanism for the second one, but
there are too many properties to fit into a 64 bit mask.

(Though the limit on 64 events might get us into trouble later...)
player: don't show the path part for external subtitle files

Show the filename only. Feature request on IRC.
manpage: explain how to replace --slave-broken
Improve setting AVOptions

Use OPT_KEYVALUELIST() for all places where AVOptions are directly set
from mpv command line options. This allows escaping values, better
diagnostics (also no more "pal"), and somehow reduces code size.

Remove the old crappy option parser (av_opts.c).
manpage: be more explicit where input.conf is located
player: allow redrawing screen during seeks

If seeks take very long, it's better not to freeze up the display.

(This doesn't handle the case when decoding video frames is extremely
slow; just if hr-seek is used, or the demuxer is threaded and blocks on
network I/O.)
player: make quit exit immediately

Stopping playback canceled waiting, but executed the remainder of the
playloop, including things like executing pointless seeks.
player: some further playloop cleanups

Handle --term-playing-msg at a better place.

Move MPV_EVENT_TICK hack into a separate function. Also add some words
to the client API that you shouldn't use it. (But better leave breaking
it for later.)

Handle --frames and frame_step differently. Remove the mess from the
playloop, and do it after frame display. Give up on the weird semantics
for audio-only mode (they didn't make sense anyway), and adjust the
manpage accordingly.
input.conf: remap 2 keys

Nobody uses 'c' (except accidentally) - remove.

Everyone agrees that OSD level cycling on 'o' is dumb, so map it to
show_progress instead. Cycling the OSD level is now available on 'O'.
No reason to ummap 'P' yet.

Also see issue #973.
gl_wayland: Don't try to resize the egl_window before it exists

If the compositor sends a configure event immediately after a window is
created (for example, if it implements tiling window management), mpv
will attempt to call wl_egl_window_resize before it has actually created
the egl_window, causing a crash.
sub: don't read packets if video and audio are disabled

Although disabling both video and audio is surely an obscure corner
case, it's allowed, and we don't want the demuxer to skip arbitrary
packets.

Basically, make the heuristic for checking interleaved files affect
external files only.
sub: fix subtitle timing for TS

The subtitle timing logic always used the demuxer's start time as video
offset. This made external subtitle files "just work" with file formats
like TS, which usually have a non-0 start time. But it was wrong for
subtitles muxed with the TS, so adjust the time offset explicitly with
external files only.
vdpau: don't crash on flipped images

It seems the vdpau API does not support these.

Do a semi-expensive emulation of it. On the other hand, it's not like
this is a commonly-used feature. (It might be better to make --vf=flip
always copy instead of flipping it via pointer tricks - but everything
allows flipped images, and even decoders or libavfilter filters could
output them.)
build: fix cross-compilation of libmpv

libmpv requires nm for creating the list of exported symbols (this is
done in syms.py). We should probably just make this list static instead,
but since this involves platform-specific code, for now this quick-fix
will do.
build: fix export of libmpv symbols on mingw

The _ usually prefixed to functions on Windows was unexpectedly missing.