Add additional kernel interfaces for setting close-on-exec on fds

when creating them: pipe2(), dup3(), accept4(), MSG_CMSG_CLOEXEC,
SOCK_CLOEXEC.  Includes SOCK_NONBLOCK support.
ok matthew@

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.89 2014/07/08 21:35:39 tedu Exp $ */
+/*	$OpenBSD: unistd.h,v 1.90 2014/08/31 01:42:36 guenther Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -484,6 +484,11 @@ int	symlinkat(const char *, int, const char *);
 int	unlinkat(int, const char *, int);
 #endif
 
+#if __BSD_VISIBLE
+int	dup3(int, int, int);
+int	pipe2(int [2], int);
+#endif
+
 #if __BSD_VISIBLE
 int	 acct(const char *);
 int	 closefrom(int);