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).

src/daemon.h

@@ -23,7 +23,7 @@
 #include <stdbool.h>
 
 void
-daemonize_init(const char *user, const char *pidfile);
+daemonize_init(const char *user, const char *group, const char *pidfile);
 
 void
 daemonize_finish(void);