notify: cleanups

* move set_nonblock{,ing}() into utils.c since we use it
elsewhere, too
* add proper error checking to set_nonblocking()
* use os_compat.h instead of individually #includ-ing system headers

git-svn-id: https://svn.musicpd.org/mpd/trunk@7217 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/notify.h

@@ -38,7 +38,7 @@ typedef struct _Notify {
 	int fds[2];
 } Notify;
 
-int initNotify(Notify *notify);
+void initNotify(Notify *notify);
 
 int waitNotify(Notify *notify);