Reworking of packet delivery to coroutines in SSH-2.

Reworking of packet delivery to coroutines in SSH-2.

Before, NULL in the dispatch table meant "send to the appropriate one of
do_ssh2_transport() and do_ssh2_authconn()".  Now those (via small
shims) are specified directly in the dispatch table, so ssh2_protocol()
is much simpler.

In the process, this has somewhat centralised the handling of gross
server protocol violations.  PuTTY will now disconnect with a rude
message when (e.g.) OpenSSH sends us an SSH_MSG_UNIMPLEMENTED when we
try to KEXINIT during authentication.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9609 cda61777-01e9-0310-a592-d414129be87e