now returns an integer: 0 means cancel the SSH connection and 1
means continue with it. Additionally, they can return -1, which
means `front end has set an asynchronous alert box in motion, please
wait to be called back with the result', and each one is passed a
callback function pointer and context for this purpose.
I have not yet done the same to askappend() yet, because it will
take a certain amount of reorganisation of logging.c.
Importantly, this checkin means the host key dialog box now works on
OS X.
[originally from svn r5330]