Warn when <malloc.h> is included, recommending <stdlib.h>

src/include/malloc.h

@@ -1,3 +1,6 @@
+/*	$OpenBSD: malloc.h,v 1.2 1996/10/12 03:13:56 tholo Exp $	*/
 /*	$NetBSD: malloc.h,v 1.3 1994/10/26 00:56:03 cgd Exp $	*/
 
+#warning "<malloc.h> is obsolete, use <stdlib.h>"
+
 #include <stdlib.h>