audio: don't list encoder AO with --audio-device=help
command: make audio device list available to the client API
input: add a function to parse mpv_node as command

For future client API enhancements.
input: use mpv_node parser for char** command parsers

Minor simplification, also drops some useless stuff.
input: cosmetics: move code
input: remove some unneeded code
etc/mplayer-input.conf: fix off by one error

playlist-pos is 0-based, but everyone thinks it's 1-based. Stupid crap.

(The "not the same as MPlayer" refers to a playlist manipulation command
that doesn't exist anymore in mpv.)

CC: @mpv-player/stable
client API: add mpv_command_node[_async]

Allows passing native types as arguments.

Also some minor doc improvements, including giving some (natural)
improvements to mpv_free_node_contents().

Note: mpv_command_node_async() is completely untested.
lua: add command_native() function

This is the Lua equivalent of mpv_command_node().