The Windows host key dialogs now have a `Help' button that should give

The Windows host key dialogs now have a `Help' button that should give
appropriate context help, iff the help file is present. (Shame it's prey to
`winhelp-crash'.)

(I've perpetrated a widening of visibility of `hwnd'; the alternative, putting
it into a frontend handle, seemed too likely to cause maintenance trouble if
we don't also _use_ that frontend handle everywhere we now use the global
`hwnd'.)

[originally from svn r5309]