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

frontend/tests/test_args.c

@@ -29,6 +29,9 @@
  */
 
 #include "testcommon.h"
+#ifndef _WIN32
+#include <sys/wait.h>
+#endif
 
 #ifdef _WIN32
 /* Hacks to stop popen() mangling input on Windows */