Move the MessageBox-with-help function out into winutils.c, although it's

Move the MessageBox-with-help function out into winutils.c, although it's
still only used for the host key popups. Side-effects:
 - requested_help is a winstuff.h global
 - Pageant now defines winstuff.h globals

(Also, my previous fix to my improved host-key dialogs only got the "changed"
case, not the "unknown" case. Some days I shouldn't be let near a keyboard.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@5415 cda61777-01e9-0310-a592-d414129be87e