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_qt/backend_vc8.pro

@@ -39,7 +39,6 @@ HEADERS +=  ../backend/aztec.h \
             ../backend/hanxin.h \
             ../backend/large.h \
             ../backend/maxicode.h \
-            ../backend/ms_stdint.h \
             ../backend/output.h \
             ../backend/pcx.h \
             ../backend/pdf417.h \