When I switch verify_ssh_host_key() and friends over to creating
non-modal message boxes and returning to the main loop, there will be
a risk that their parent window will need to close for some other
reason while the user hasn't answered the pending question yet. (E.g.
if the user presses the main session window's close button, which will
no longer be a prohibited UI action once the transient dialog is not
modal.)
At that point we need to get rid of the pending dialog box, both for
UI purposes (it would look silly and be confusing to leave it lying
around) and for memory management (if the user subsequently clicks OK
in such a dialog it would probably try to leave its result somewhere
stale).
So now there's a mechanism for gtkwin.c remembering what the current
'network prompt dialog' is, if any (in which category I intend to
include everything triggered from ssh.c's various reasons for asking
crypto-related questions), and cleaning it up when the struct gui_data
it belongs to goes away.
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