Track source address and port on UDP requests (svc_dg).

WARNING!  This is a significant API change, as the svc_dg
caller is required to free rq_rtaddr, if assigned.

The decision to to store this information in an opaque form
might not be ideal, but is consistent with current transport
handle logic, so makes sense for now.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>