Sensibly enforce non-interactive rekeying.

Sensibly enforce non-interactive rekeying.

We now only present the full set of host key algorithms we can handle
in the first key exchange. In subsequent rekeys, we present only the
host key algorithm that we agreed on the previous time, and then we
verify the host key by simply enforcing that it's exactly the same as
the one we saw at first and disconnecting rudely if it isn't.

git-svn-id: http://svn.tartarus.org/sgt/putty@10027 cda61777-01e9-0310-a592-d414129be87e