Validate.sol should verify each signature individually

[smartcontracts]

Validate.sol currently assumes that both signatures on the inputs to a transaction will come from msg.sender. However, it's possible that the signatures come from another user or two different users entirely. Instead of assuming msg.sender, we should require that the owners of the signatures be provided.

Here's an example:

    /**
     * @dev Checks if the signatures on a transaction are valid
     * @param txHash Hash of the transaction to check
     * @param blknum1 Block number of the first input
     * @param owner1 Address the first signature should belong to
     * @param blknum2 Block number of the second input
     * @param owner2 Address the second signature should belong to
     * @param sigs Signatures to verify
     * @return true if the transaction is valid, false otherwise
     */
    function checkSigs(bytes32 txHash, uint256 blknum1, address owner1, uint256 blknum2, address owner2, bytes sigs)
        internal
        view
        returns (bool)
    {
        require(sigs.length % 65 == 0 && sigs.length <= 260);

        bytes memory sig1 = ByteUtils.slice(sigs, 0, 65);
        bytes memory sig2 = ByteUtils.slice(sigs, 65, 65);

        bool sig1valid = true;
        bool sig2valid = true;
        if (blknum1 > 0) {
            sig1valid = ECRecovery.recover(txHash, sig1) == owner1;
        } 
        if (blknum2 > 0) {
            sig2valid = ECRecovery.recover(txHash, sig2) == owner2;
        }

        return sig1valid && sig2valid;
    }

[smartcontracts]

Meant to put this into the no-confs branch.