Revamp interface to verify_ssh_host_key() and askalg(). Each of them

Revamp interface to verify_ssh_host_key() and askalg(). Each of them
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.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5330 cda61777-01e9-0310-a592-d414129be87e