- correct brk(2) prototype and man page to match implementation and POSIX.

brk(2) returns an int value not a void *. brk(2) returns 0 upon success not
a pointer to the new end of memory. okay millert@, kettenis@, deraadt@

src/include/unistd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: unistd.h,v 1.59 2006/12/20 08:12:00 espie Exp $ */
+/*	$OpenBSD: unistd.h,v 1.60 2008/04/24 20:43:20 kurt Exp $ */
 /*	$NetBSD: unistd.h,v 1.26.4.1 1996/05/28 02:31:51 mrg Exp $	*/
 
 /*-
@@ -176,7 +176,7 @@ int	 ttyname_r(int, char *, size_t)
 
 #if __BSD_VISIBLE ||  __XPG_VISIBLE <= 500
 /* Interfaces withdrawn by X/Open Issue 5 Version 0 */
-void	 *brk(void *);
+int	 brk(void *);
 int	 chroot(const char *);
 int	 getdtablesize(void);
 int	 getpagesize(void);