Get rid of malloc.h.

This is a non-POSIX/ISO-C header.  It is ok to include this on Linux, but it
is obsolete on BSD; FreeBSD even throws an error if you include it with
__STDC__ defined.  Every system should nowadays have malloc() defined in
stdlib.h.

Diff is largely mechanical, replacing malloc.h with stdlib.h where it is not
yet included anyway.

cde/lib/DtHelp/GifUtils.c

@@ -67,7 +67,7 @@
 /* include files */
 #include <stdio.h>
 #include <stdarg.h>
-#include <malloc.h>
+#include <stdlib.h>
 #include <string.h>
 #include <math.h>
 #include <assert.h>