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.

 * 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://").

git_remote_set_transport now takes a transport factory rather than a transport
git_clone_options now allows the caller to specify a remote creation callback