input: "quit_watch_later" and "stop" are abort commands

This means they get special handling for asynchronously aborting
playback, even if the player is "stuck".

Also document "stop". It seems somewhat useful for client API users
(although that will be implemented properly only in the following
commits.)
stream: redo playback abort handling

This mechanism originates from MPlayer's way of dealing with blocking
network, but it's still useful. On opening and closing, mpv waits for
network synchronously, and also some obscure commands and use-cases can
lead to such blocking. In these situations, the stream is asynchronously
forced to stop by "interrupting" it.

The old design interrupting I/O was a bit broken: polling with a
callback, instead of actively interrupting it. Change the direction of
this. There is no callback anymore, and the player calls
mp_cancel_trigger() to force the stream to return.

libavformat (via stream_lavf.c) has the old broken design, and fixing it
would require fixing libavformat, which won't happen so quickly. So we
have to keep that part. But everything above the stream layer is
prepared for a better design, and more sophisticated methods than
mp_cancel_test() could be easily introduced.

There's still one problem: commands are still run in the central
playback loop, which we assume can block on I/O in the worst case.
That's not a problem yet, because we simply mark some commands as being
able to stop playback of the current file ("quit" etc.), so input.c
could abort playback as soon as such a command is queued. But there are
also commands abort playback only conditionally, and the logic for that
is in the playback core and thus "unreachable". For example,
"playlist_next" aborts playback only if there's a next file. We don't
want it to always abort playback.

As a quite ugly hack, abort playback only if at least 2 abort commands
are queued - this pretty much happens only if the core is frozen and
doesn't react to input.
input: distinguish playlist navigation and quit commands for abort

Refine the ugly hack from the previous commit, and let the "quit"
command and some others abort playback immediately. For
playlist_next/playlist_prev, still use the old hack, because we can't
know if they would stop playback or not.
client API: use playback abort mechanism

If you send the "quit" or "stop" command with the client API, it will
now attempt to kill network I/O immediately (same as normal input in the
previous commits).