The back ends now contain their own copies of the Config structure,

The back ends now contain their own copies of the Config structure,
and have a function to pass in a new one. (Well, actually several
back ends don't actually bother to do this because they need nothing
out of Config after the initial setup phase, but they could if they
wanted to.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@2561 cda61777-01e9-0310-a592-d414129be87e