Install trigger for DAO-rescue soft-fork.

Soft-fork should only be primed to trigger if the gas-limit of
block #1760000 is at most 4,000,000.

To accomplish this we pass in the gas limit of that block to
EnvInfo so it can inform Schedule. This gets marshalled through
`OpenBlock`/`ClosedBlock` and the `enact` functions much like
`last_hashes`. `block.rs`'s `env_info()` takes care to ensure
that if the current block happens to be #1760000, then we
populate with the current `gas_limit`.