WIP: Define IPV6_TCLASS when it isn't defined on Hurd
As a followup to !30 (closed), this is an attempt to define the right value for the IPV6_TCLASS socket option when the system headers don't give it to us.
Based on the discussion on a gpsd Debian bug report.
The previous change c04dd44b is left in for now, but can also be reverted if it would be preferable to fail at build time and properly handle any systems that don't define the option.