mpd 0.18.13

Bug-release fix with the following changes:
* protocol
  - don't change song on "seekcur" in random mode
* decoder
  - dsdiff, dsf: fix endless loop on malformed file
  - ffmpeg: support ffmpeg/libav version 11
  - gme: fix song duration
* output
  - alsa: fix endless loop at end of file in dsd_usb mode
* fix state file saver
* fix build failure on Darwin