Make the composite 'connect to server' code useful for security=server

Make the composite 'connect to server' code useful for security=server

The ability to short-circuit the connection code to only do a negprot
allows us to do the rest once we have the user's password.  We return
the 8 byte challenge so we can pass it to the client.

Andrew Bartlett
(This used to be commit 40fe386b0374df8b390b995c332d048dbbc08f1b)