Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.

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 should make the mingw compiler happy.

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.

If the certificate validation fails (or always in the case of ssh),
let the user decide whether to allow the connection.

The data structure passed to the user is the native certificate
information from the underlying implementation, namely OpenSSL or
WinHTTP.

 * Move the transport registration mechanisms into a new header under
   'sys/' because this is advanced stuff.
 * Remove the 'priority' argument from the registration as it adds
   unnecessary complexity.  (Since transports cannot decline to operate,
   only the highest priority transport is ever executed.)  Users who
   require per-priority transports can implement that in their custom
   transport themselves.
 * Simplify registration further by taking a scheme (eg "http") instead
   of a prefix (eg "http://").