Make comments more consistent

- Changes most `/* comments */` to `// comments`
- Changes `/**` block comments consistently to `/*`
- Adds consistent license comments to all files

Also renames `T_POP_SET` to `T_Z80_SET`

include/asm/opt.h

@@ -22,4 +22,4 @@ void opt_Parse(char const *option);
 void opt_Push(void);
 void opt_Pop(void);
 
-#endif
+#endif // RGBDS_OPT_H