mpd 0.19.4

This is a bugfix release:
* protocol
  - workaround for buggy clients that send "add /"
* decoder
  - ffmpeg: support opus
  - opus: add MIME types audio/ogg and application/ogg
* fix crash on failed filename charset conversion
* fix local socket detection from uid=0 (root)
mpd: update 0.19.4 bottle.