Standardise header include guards

Tokens that begin with two underscores __* or an underscore and an uppercase letter _X* are reserved,
so we should avoid those for our own include guards.  The standard I'm settling on for foo.h is FOO_H_.

include/screen.h

@@ -9,9 +9,8 @@
  *
  * @(#)$Id$
  */
-
-#ifndef __screen_h_
-#define __screen_h_
+#ifndef SCREEN_H_
+#define SCREEN_H_
 
 #include "window.h"
 
@@ -77,4 +76,4 @@ extern  Window	*debugging_window;
 
 extern	int	strip_ansi_never_xlate;
 
-#endif /* __screen_h_ */
+#endif /* SCREEN_H_ */