Fix for communicating with Pageant from a cygwin process

As I mentioned previously, Pageant's security measures reject messages coming from cygwin processes (and their children). I submitted a patch that fixes this for charade. The trick is to copy the security descriptor from the Pageant process so they'll be an exact match. Any app should be able to use this same technique.

Sadly, the Putty folks have failed to incorporate the change into winpgntc.c.