Add context help to a couple of message boxes. Unfortunately the ones

Add context help to a couple of message boxes. Unfortunately the ones
I wanted to get to -- "software caused connection abort" and friends --
are going to be more involved (probably requiring some cross-platform
notion of help contexts), and these ones hardly seem worth the effort.
Still, I've done it now.

Side-effect: Pageant now uses the same `hinst' and `hwnd' globals as
everything else. Tested basic functionality.

[originally from svn r5417]