Log Message:
libmpdclient: update to 2.16.

libmpdclient 2.16 (2018/10/09)
* support MPD protocol 0.19
  - "mount", "unmount", "listmounts"
* support MPD protocol 0.17
  - "load" with range
* add mpd_output_get_attribute()
* use UINX_MAX instead of `(unsigned)-1` to avoid undefined behavior

libmpdclient 2.15 (2018/09/02)
* support MPD protocol 0.21
  - search: support search expressions
  - search: descending sort
  - search: support sorting by "Last-Modified"
  - song: audio format
* support MPD protocol 0.17
  - player: command "seekcur"
* fix `-Wrestrict` compiler warning