libmpdclient: update to 2.20

libmpdclient 2.20 (2021/11/23)
* support MPD protocol 0.23
  - getvol
  - position argument for searchadd, findadd, load and playlistadd
  - whence argument for addid and add
  - range argument for playlistdelete
* support MPD protocol 0.22.4
  - binarylimit
* support MPD protocol 0.22.0
  - readpicture
* support MPD protocol 0.21.0
  - albumart
* support tags "ComposerSort", "Ensemble", "Movement",
  "MovementNumber", "Location"
* queue: make "priority" parameter unsigned

(nia)