Backport rred patches from my own sid branch to the 0.7.25 branch

Backport rred patches from my own sid branch to the 0.7.25 branch

* rewrite and refactor rred method to be able to handle even big (>30 MB)
  patches (Closes: #554349) and hardening the method itself by using more
  constants and a return value which can't be misinterpreted as linenumber
* Finally adope the patch from Morten Hustveit <morten@debian.org> to be
  able to optional use mmaps and iovec to increase patch speed -
  but as this increase memory usage we can always fall back to the "old"
  method which doesn't depend on mmaps.