AddrSplosion -- Partie Deux Avec Vengeance

Implement io.Seeker on unixio.DAGReader

dropped down log.Errors

misc user-friendliness fixes

CLI changes

NAT-Lite

should be returning false.

Exposing the NAT to the core is unnecessary. The Host can take
care of it. If a need emerges, we can address it then.

- allow retrieving addresses
- allow notifications on mapping changes
- allow lifecycle mgmt (future commit will manage it)

cmd/ipfs: On command ClientError, suggest checking helptext instead of showing short helptext. Resovles #630