Adding of plchangesposid command, this is a stripped down version of the plchanges command, it only sends the pos and id of the changed command. Libmpd allready has support for it, and libmpdclient in libmpd too

git-svn-id: https://svn.musicpd.org/mpd/trunk@4101 09075e82-0dd4-0310-85a5-a0d7c8717e4f