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

LICENSE

@@ -0,0 +1,13 @@
+ZINT was originally developed by Robin Stuart and licensed under the terms of the GPL (Gnu
+Public License). In 2013 Robin Stuart and all developers that contributed to the code agreed
+to change the license to a less restrictive one (see ZINT mailing list of May 2013 for
+related mails) under the following conditions:
+- the names of original copyright holder Robin Stuart and contributors are not removed,
+  neither from sources nor from the manual
+- the documentation is done in British English (as Robin said: "Oh, and please don't
+  replace the word "colour" with "color" in the documentation - that really annoys me!")
+
+Change to BSD-license is done for backend and therefore for ZINT shared library only, for
+the frontends and Qt4-backend the GPL is still valid. Since BSD-license is GPL-compatible
+this gives the possibility to include ZINT library in own products or link against it from
+own software.