Log Message:
musicpd: update to 0.20.19.

Switch to ffmpeg4.

ver 0.20.19 (2018/04/26)
* protocol
  - validate absolute seek time, reject negative values
* database
  - proxy: fix "search already in progress" errors
  - proxy: implement "list ... group"
* input
  - mms: fix lockup bug and a crash bug
* decoder
  - ffmpeg: fix av_register_all() deprecation warning (FFmpeg 4.0)
* player
  - fix spurious "Not seekable" error when switching radio streams
* macOS: fix crash bug