Sync readlink(2) with IEEE Std 1003.1-2008.

discussion, help and ok guenther@

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.91 2014/08/31 04:02:08 guenther Exp $ */
+/*	$OpenBSD: unistd.h,v 1.92 2014/09/01 05:09:52 doug Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -424,7 +424,7 @@ int	 lchown(const char *, uid_t, gid_t);
 int	 mkstemp(char *);
 char	*mktemp(char *);
 int	 nice(int);
-int	 readlink(const char *, char *, size_t)
+ssize_t readlink(const char * __restrict, char * __restrict, size_t)
 		__attribute__ ((__bounded__(__string__,2,3)));
 int	 setkey(const char *);
 int	 setpgrp(pid_t pid, pid_t pgrp);	/* obsoleted by setpgid() */