mpd 0.18.12

New release logs are as follows:
* database
  - proxy: fix build failure with libmpdclient 2.2
  - proxy: fix add/search and other commands with libmpdclient < 2.9
* decoder
  - audiofile: improve responsiveness
  - audiofile: fix WAV stream playback
  - dsdiff, dsf: fix stream playback
  - dsdiff: fix metadata parser bug (uninitialized variables)
  - faad: estimate song duration for remote files
  - sndfile: improve responsiveness

* randomize next song when enabling "random" mode while not playing
* randomize next song when adding to single-song queue