We don't need the remote loaded, and the function extracted both of
these from the git_remote in order to do its work, so let's remote a
step and not ask for the loaded remote at all.

This fixes #2390.

Provide a callback for certificate validation

The user may have the data hashed as MD5 or SHA-1, so we should provide
both types for consumption.

Instead of using the libssh2 defines, provide our own, which eases usage
as we do not need to check whether libgit2 was built with libssh2 or not.

Instead of spreading the data in function arguments, some of which
aren't used for ssh and having a struct only for ssh, use a struct for
both, using a common parent to pass to the callback.

This option make it easy to ignore anything about the server we're
connecting to, which is bad security practice. This was necessary as we
didn't use to expose detailed information about the certificate, but now
that we do, we should get rid of this.

If the user wants to ignore everything, they can still provide a
callback which ignores all the information passed.

This should make the mingw compiler happy.

We should let the user decide whether to cancel the connection or not
regardless of whether our checks have decided that the certificate is
fine. We provide our own assessment to the callback to let the user fall
back to our checks if they so desire.

Instead of the parsed data, we can ask OpenSSL to give us the
DER-encoded version of the certificate, which the user can then parse
and validate.