I've switched it to using the new non-modal create_message_box, and
provided a callback function which handles the cleanup afterwards.
I had expected this to be a lot more work, because I'd imagined that
I'd have to contort the coroutines in ssh.c to give them the ability
to wait for an asynchronously delivered result from that user prompt.
But in fact that wasn't necessary, because just such a mechanism has
been sitting there unused since commit 8574822b9 in 2005, when I added
it as part of my _previous_ attempt to write an OS X front end! (The
abandoned one written in native ObjC + Cocoa.)
624f5b7d47