Improve #include guard macros

Avoid using reserved identifiers such as `_DOHINT_H` (fixes #1989),
fix cases where the name doesn't match the filename, and make the naming
more consistent and less likely to collide with include guards in other
headers.

Examples/javascript/nspace/example.h

@@ -1,5 +1,5 @@
-#ifndef _example_guardian_
-#define _example_guardian_
+#ifndef EXAMPLE_H
+#define EXAMPLE_H
 
 int module_function() { return 7; }
 int module_variable = 9;