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`.

ethcore/src/miner/miner.rs

@@ -266,6 +266,7 @@ impl MinerService for Miner {
 					last_hashes: last_hashes,
 					gas_used: U256::zero(),
 					gas_limit: U256::max_value(),
+					dao_rescue_block_gas_limit: chain.dao_rescue_block_gas_limit(),
 				};
 				// that's just a copy of the state.
 				let mut state = block.state().clone();