Add mpd and add or bump necessary deps

Hey, I noticed mpd lying around in cross, but not in spk so here it is. I've wanted mpd on my NAS for quite a while since I wasn't satisfied with either ds audio or plex, and I believe this makes for quite a usable mpd already. Currently supported features:

Decoders plugins:
 [mad] mp3 mp2
 [vorbis] ogg oga
 [oggflac] ogg oga
 [flac] flac
 [sndfile] wav aiff aif au snd paf iff svx sf voc w64 pvf xi htk caf sd2
 [audiofile] wav au aiff aif
 [dsdiff] dff
 [dsf] dsf
 [faad] aac
 [mp4ff] m4a m4b mp4
 [pcm]

Output plugins:
 null fifo alsa oss httpd recorder

Encoder plugins:
 null vorbis wave flac

Input plugins:
 file curl

Playlist plugins:
 extm3u m3u xspf pls asx rss cue cue

Protocols:
 file:// http://

People might wanna have wavpack and opus decoders, as well as lame (silly me, that one is in cross and I forgot to include it) and opus encoders, I can get on it if needed. As for Monkey's Audio decoding, I believe this can only be achieved via ffmpeg atm, bringing that in would be a bit more work, but why not.

The config I included uses most defaults, and sets up an httpd output on port 8000, codec is vorbis q5. Maybe we could add a fancy wizard to change some options, but there are quite a few of them, and directly editing the conf file via ssh (or with the config file editor package) makes more sense imho as the file is very verbose.

What do you think?