Provide an 'extra' pointer in ssh_signkey and ssh_kex.

Provide an 'extra' pointer in ssh_signkey and ssh_kex.

This gives families of public key and kex functions (by which I mean
those sharing a set of methods) a place to store parameters that allow
the methods to vary depending on which exact algorithm is in use.

The ssh_kex structure already had a set of parameters specific to
Diffie-Hellman key exchange; I've moved those into sshdh.c and made
them part of the 'extra' structure for that family only, so that
unrelated kex methods don't have to faff about saying NULL,NULL,0,0.
(This required me to write an extra accessor function for ssh.c to ask
whether a DH method was group-exchange style or fixed-group style, but
that doesn't seem too silly.)