src: major cleanups, introduce new eXtended Span class

- initial minimally invasive eXtended Span implementation
- rename ptr_diff to ptr_diff_bytes
- move some files to util subdir
- lots of cleanups
- start using the new checked pointers - this needs some real-world testing

src/options.h

@@ -25,8 +25,9 @@
    <markus@oberhumer.com>               <ezerotven+github@gmail.com>
  */
 
-#ifndef __UPX_OPTIONS_H
-#define __UPX_OPTIONS_H 1
+#pragma once
+#ifndef UPX_OPTIONS_H__
+#define UPX_OPTIONS_H__ 1
 
 /*************************************************************************
 // globals