test, cmd/evm, core, core/vm: illegal code hash implementation

This implements a generic approach to enabling soft forks by allowing
anyone to put in hashes of contracts that should not be interacted from.
This will help "The DAO" in their endevour to stop any whithdrawals from
any DAO contract by convincing the mining community to accept their code
hash.

cmd/evm/main.go

@@ -220,6 +220,7 @@ type ruleSet struct{}
 
 func (ruleSet) IsHomestead(*big.Int) bool { return true }
 
+func (self *VMEnv) MarkCodeHash(common.Hash)   {}
 func (self *VMEnv) RuleSet() vm.RuleSet        { return ruleSet{} }
 func (self *VMEnv) Vm() vm.Vm                  { return self.evm }
 func (self *VMEnv) Db() vm.Database            { return self.state }