Replace NULL macro defs with null.h include

commands/elvis/opts.c

@@ -15,9 +15,7 @@
 #include "config.h"
 #include "vi.h"
 #include "ctype.h"
-#ifndef NULL
-#define NULL (char *)0
-#endif
+#include <sys/null.h>
 extern char	*getenv();
 
 /* maximum width to permit for strings, including ="" */

commands/postmort/postmort.c

@@ -45,7 +45,6 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
-#undef NULL
 #include <string.h>
 #include <unistd.h>