media-sound/mpd: add a systemd user service

This allows a user to run mpd through systemd locally, instead of a
system-wide global instance under the mpd user.

media-sound/mpd/mpd-0.19.11.ebuild

@@ -223,6 +223,8 @@ src_install() {
 
 	newinitd "${FILESDIR}"/${PN}2.init ${PN}
 
+	systemd_newuserunit systemd/${PN}.service ${PN}.service
+
 	if use unicode; then
 		sed -i -e 's:^#filesystem_charset.*$:filesystem_charset "UTF-8":' \
 			"${ED}"/etc/mpd.conf || die "sed failed"