Pass the ssh_signkey structure itself to public key methods.

Pass the ssh_signkey structure itself to public key methods.

Not all of them, but the ones that don't get a 'void *key' parameter.
This means I can share methods between multiple ssh_signkey
structures, and still give those methods an easy way to find out which
public key method they're dealing with, by loading parameters from a
larger structure in which the ssh_signkey is the first element.

(In OO terms, I'm arranging that all static methods of my public key
classes get a pointer to the class vtable, to make up for not having a
pointer to the class instance.)

I haven't actually done anything with the new facility in this commit,
but it will shortly allow me to clean up the constant lookups by curve
name in the ECDSA code.