Separate key-type enum values for old and new OpenSSH keys.

Separate key-type enum values for old and new OpenSSH keys.

It's all very well for these two different formats to share a type
code as long as we're only loading them and not saving, but as soon as
we need to save one or the other, we'll need different type codes
after all.

This commit introduces the openssh_new_write() function, but for the
moment, it always returns failure.