Asynchronous agent requests on Windows. Actually, I've kept the

Asynchronous agent requests on Windows. Actually, I've kept the
ability to do synchronous ones as well, because PSCP and PSFTP don't
really need async ones and it would have been a serious pain to
implement them. Also, Pageant itself when run as a client of its
primary instance doesn't benefit noticeably from async agent
requests.

git-svn-id: http://svn.tartarus.org/sgt/putty@3154 cda61777-01e9-0310-a592-d414129be87e