ao_alsa: print channel map if setting it fails

This message is printed when the audio device advertised a channel map,
but couldn't set it - which is probably a dmix bug (we'll never know,
ALSA doesn't take bug reports).

Print the requested map, so that the user (maybe) can make a connection
when seeing the message and the actually used channel map, which might
be less confusing. Or at least less useless.
ao_portaudio: remove this audio output

It's just completely useless. We have good native support for all 3
desktop platforms, and ao_sdl or ao_openal as fallbacks.
player: make --shuffle/--merge-files affect runtime loaded playlists

Until now, these options took effect only at program start. This could
be confusing when e.g. doing "mpv list.m3u --shuffle". Make them always
take effect when a playlist is loaded either via a playlist file, or
with the "loadlist" command.
player: filter tags, add --display-tags option

This attempts to increase user-friendliness by excluding useless tags.
It should be especially helpful with mp4 files, because the FFmpeg mp4
demuxer adds tons of completely useless information to the metadata.

Fixes #1403.
command: add filtered-metadata property

Goes in hand with the previous commit; the main purpose is to easily
show on OSD what is shown on terminal.
demux_mf: move mf.c contents to demux_mf.c

There's no reason why parts of this demuxer would be in a separate
source file. The existence of this code is already somewhat questionable
anyway, so it may as well be dumped into a single file.

Even stranger that demux.c included mf.h for no reason (it was an
artifact from 2002 when the architecture was uncleaner).
demux_mkv: reduce log noise

This message can happen a lot for mkv files which index clusters in the
seekhead (which is also broken non-sense, but that's a different story).

Also remove a duplicate define from matroska.h.
rpi: initial commit

Setup the audio decoder, video decoder and video output basic code.

Add README on external dependency on pkg-config files for the firmware.

Add --rpi feature to the build system.
rpi: add list of supported video decoder codecs
rpi: fix building
rpi: add some audio decoders
rpi: remove some leftovers from the README
rpi: add omx common helpers
rpi: add license headers
rpi: add some omx helpers
WIP