Add dirfd() as a function to libc per POSIX requirement; dirfd() macro

to be pruned later when DIR is made an opaque type.
ok guenther@; prodding by brad@ for VLC and other ports

src/include/dirent.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: dirent.h,v 1.24 2011/07/18 17:29:49 matthew Exp $	*/
+/*	$OpenBSD: dirent.h,v 1.25 2012/03/22 01:13:40 matthew Exp $	*/
 /*	$NetBSD: dirent.h,v 1.9 1995/03/26 20:13:37 jtc Exp $	*/
 
 /*-
@@ -119,6 +119,9 @@ int scandir(const char *, struct dirent ***,
     int (*)(struct dirent *), int (*)(const void *, const void *));
 int alphasort(const void *, const void *);
 #endif
+#if __POSIX_VISIBLE >= 200809 || __XPG_VISIBLE > 600
+int (dirfd)(DIR *);
+#endif
 __END_DECLS
 
 #endif /* !_KERNEL */