First attempt at a platform-independent keyboard handler. This isn't complete

First attempt at a platform-independent keyboard handler.  This isn't complete
yet -- there's no Alt+keypad support, and no way for the front-end to find
out what it should do with the Num Lock light.  It's also not fully tested.
Nonetheless, it's at least as good as the previous Mac keyboard handler.
Other platforms probably shouldn't adopt it just yet.

git-svn-id: http://svn.tartarus.org/sgt/putty@2728 cda61777-01e9-0310-a592-d414129be87e