Provide host name to certificate_check_cb

Verifying a certificate also requires the hostname of the host we're connecting to, thus, provide host name to certificate_check_cb
The ambiguity argument makes sense, telling the user "this is the DNS lookup I made", especially if we ever support reading the ssh configuration. I'm still strongly against using libgit2 as anything close to a url parsing library.

But if we're going to do that, it shouldn't be in the cert info, because it's not cert info. If we pass it, it should be as a function argument.