Begin destabilisation in the wake of 0.53! This checkin contains the

Begin destabilisation in the wake of 0.53! This checkin contains the
beginning of a Unix port. It's nowhere near done, and currently it
won't even compile on Unix. But this represents the start of the
process of separating out platform-specific code, and also contains
the mkfiles.pl changes required to support a Unix makefile and a
non-flat source tree.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1993 cda61777-01e9-0310-a592-d414129be87e