Restructure KEXINIT generation and parsing.

Restructure KEXINIT generation and parsing.

The new code remembers the contents and meaning of the outgoing KEXINIT
and uses this to drive the algorithm negotiation, rather than trying to
reconstruct what the outgoing KEXINIT probably said.  This removes the
need to maintain the KEXINIT generation and parsing code precisely in
parallel.

It also fixes a bug whereby PuTTY would have selected the wrong host key
type in cases where the server gained a host key type between rekeys.