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
I agree; cert validation is place where I want as little ambiguity between what I think the user entered and what libgit2 is connecting to as possible.  If my parsing is broken (or libgit2's is, or we're just different in some way) that opens up an interesting attack vector.)