mpd: big overhaul of deps, options

Add sqlite3 and libsamplerate as depends; they're small, and there's
really no reason not to want them.  Remove an unnecessary dep on mad
(ffmpeg can do everything it can).

Add :optional support for WavPack, JACK, and two-lame.

Make the deps for streaming output (libshout, lame, flac) optional.

Rename "--enable-soundcloud" to "--with-yajl" which, while less
informative, correctly controls the dependency on yajl.

Rename "--libwrap" to "--with-libwrap". Change its description to
reflect how it behaves differently on OSX <10.7, 10.7; remove on 10.8,
where it's useless.

Rename "--lastfm" to "--enable-lastfm" to follow the
options-start-with-verb conveition.

Document all of the options changes/quirks in caveats.
mpd: bump to 0.17.4