For such a path '/~/...' the leading '/' is stripped so the server will
get a path starting with '~' and correctly handle it.

Add a custom param to git_smart_subtransport_definition

In case of a bad url or other error during the connection setup, we
close the stream via free.

The smart transport has already take the payload param. For the
sub transport a payload param is useful for the implementer.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

 * 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://").

This continues auditing all the places where GIT_EUSER is being
returned and making sure to clear any existing error using the
new giterr_user_cancel helper.  As a result, places that relied
on intercepting GIT_EUSER but having the old error preserved also
needed to be cleaned up to correctly stash and then retrieve the
actual error.

Additionally, as I encountered places where error codes were not
being propagated correctly, I tried to fix them up.  A number of
those fixes are included in the this commit as well.