Introduce framework for authenticating with the local X server.

Windows and Mac backends have acquired auth-finding functions which
do nothing; Unix backend has acquired one which actually works, so
Plink can now do X forwarding believably.
(This checkin stretches into some unlikely parts of the code because
there have been one or two knock-on effects involving `const'. Bah.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@2536 cda61777-01e9-0310-a592-d414129be87e