Add NSEC3 helper function back in

I'm going to use this in 'q', but it may came in handy for other
developers too. This is the first ramp up to a full blown Go only
recursive resolver.
Add NSEC prototype
Add Proof function for NSEC/NSEC3
Add NSEC3 NXDOMAIN proof
Fix NSEC3 Proof tests
Rename Proof to VerifyNameError
... and in the tests too
Split off stuff to package chain

Create a resolver package called chain, to keep in the spririt
of Bind, Unbound, Drill and Knot (and maybe others)
Merge branch 'nsec'