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/history.h

@@ -7,9 +7,8 @@
  *
  * @(#)$Id$
  */
-
-#ifndef __history_h__
-#define __history_h__
+#ifndef HISTORY_H_
+#define HISTORY_H_
 
 	BUILT_IN_COMMAND(history);
 	void	set_history_size	(Window *, char *, int);
@@ -23,4 +22,4 @@
 #define NEXT 0
 #define PREV 1
 
-#endif /* _HISTORY_H_ */
+#endif /* HISTORY_H_ */