Support reconfiguration of key exchange in mid-session. The fiddly

Support reconfiguration of key exchange in mid-session. The fiddly
bit is working out when to reschedule the next rekey for when the
timeout or data limit changes; sometimes it will be _right now_
because we're already over the new limit.

Still to do: the Kex panel should not appear in mid-session if we
are using SSHv1.

git-svn-id: http://svn.tartarus.org/sgt/putty@5031 cda61777-01e9-0310-a592-d414129be87e