DAO hard-fork (#1483)

* Minor additions to allow resetting of code.

* Add test.

* Provisional DAO hard-fork proposal.

* Change to reflect latest HF spec.

* Include extradata restrictions and overrides.

* Introduce new tests.

* Update tests to new spec format.

* Allow JSON chain spec fields to be optional.

* Remove superfluous definitions. Fix overflow risk.

* Fix build.

* Add missing file.

* Remove old flag.

* Update to latest address set.

* Update tests and test spec to latest.

Change the mining default to release only on own transactions.