This was harder than verify_ssh_host_key() and askalg() put
together, because:
(a) askappend() can be called at any time, since it's a side effect
of data-logging functions. Therefore there can be an unfinished
askappend() alert at any time, and hence the OS X front end has
to be prepared to _queue_ other alerts which occur during that
time.
(b) logging.c has to do something with data that comes in while
it's waiting for an answer to askappend(). It buffers it until
it knows what the user wants done with it. This involved
something of a reorganisation of logging.c.
[originally from svn r5344]