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/help.h

@@ -7,10 +7,9 @@
  *
  * @(#)$Id$
  */
-
-#ifndef __help_h
-# define __help_h
+#ifndef HELP_H_
+#define HELP_H_
 
 	void	help (char *, char *, char *, char *);
 
-#endif /* __help_h */
+#endif /* HELP_H_ */