backend: define z_alloca() and use for both Unix and Windows;

replace double-slash comments with old-skool slash asterisk ones;
  define uint16_t etc for Windows ourselves and remove ms_stdint.h &
  stdint_msvc.h as no longer used;
  (backend (excepting test suite) now C89 compatible)
LICENSE: move from backend to root and move COPYING to frontend, with
  copies in frontend_qt & backend_qt, so in where it applies;
  add LICENSE section from manual to root README

backend/large.h

@@ -34,12 +34,6 @@
 #ifndef Z_LARGE_H
 #define Z_LARGE_H
 
-#ifndef _MSC_VER
-#include <stdint.h>
-#else
-#include "ms_stdint.h"
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif /* __cplusplus */