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.