Security audit issues fixed (#1279)

* Restrict network key file permissions

* Check for overflow in str to bigint conversion

* RLP decoder overflow check

parity/signer.rs

@@ -18,7 +18,7 @@ use std::io;
 use std::path::PathBuf;
 use std::sync::Arc;
 use util::panics::{PanicHandler, ForwardPanic};
-use util::keys::directory::restrict_permissions_owner;
+use util::path::restrict_permissions_owner;
 use die::*;
 use rpc_apis;