test/test_protocol: add missing stdlib.h include

EXIT_SUCCESS and EXIT_FAILURE are defined in stdlib.h, not unistd.h.
D'oh!
playlist/embcue: change name string to "embcue"

The name "cue" was listed twice in "mpd --version".
configure.ac: add option to disable the DSD decoders

Allow building a smaller MPD binary for people who don't need DSD.
CommandLine: update copyright year
configure.ac: allow building MPD without output plugin

MPD can easily be used as a database provider for the proxy database
plugin.  In that case, it needs only one "null" output, and no real
output plugin.
configure.ac: allow building MPD without decoder plugin

There's always the "PCM" decoder plugin, which was never checked by
configure.ac.
release v0.18.15
configure.ac: prepare for 0.18.16
configure.ac: fix DSD breakage due to typo
release v0.18.16
Merge tag 'v0.18.16'
output/alsa: change a few "dsd" to "dop"