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/externalities.rs

@@ -318,7 +318,8 @@ mod tests {
 			difficulty: 0.into(),
 			last_hashes: vec![],
 			gas_used: 0.into(),
-			gas_limit: 0.into()
+			gas_limit: 0.into(),
+			dao_rescue_block_gas_limit: None,
 		}
 	}