daemon: added "group" configuration option

The "group" configuration option is similar to "user" as it
sets user set what group MPD shall run as.  With "user"
option, MPD changed GID to the GID of the user, however,
more control could be desired.

Moreover, the patch changes the way of checking whether no
setuid(2)/setgid(2) is required -- previously user names
were compered, now UID and GIDs are compered (ie. the one we
already have (getuid(2)/getgid(2)) with the one we want to
change to).

doc/mpdconf.example

@@ -61,6 +61,13 @@
 #
 #user				"nobody"
 #
+# This setting specifies the group that MPD will run as. If not specified
+# primary group of user specified with "user" setting will be used (if set).
+# This is useful if MPD needs to be a member of group such as "audio" to
+# have permission to use sound card.
+#
+#group				"nogroup"
+#
 # This setting sets the address for the daemon to listen on. Careful attention
 # should be paid if this is assigned to anything other then the default, any.
 # This setting can deny access to control of the daemon.