Start using notify_remote_exit() in the Windows front end, in place

Start using notify_remote_exit() in the Windows front end, in place
of the previous ad-hockery which depended on the return value from
select_result() and hence which will not adapt sensibly to a world
in which the primary session is something local rather than a
network connection.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6802 cda61777-01e9-0310-a592-d414129be87e