Proof of ownership for proxy anonymous account

[xlc]

Currently it is hard for a service to verify ownership of an address. One way is have the address sign a message using private key, which does not work for proxy anonymous address or multi-sig address.

system.remark can be a solution by proof the ability to send transaction from an address, however it still have some limitations:

• No event, so there is no easy way to verify if the execution succeeded. Image a A -> B -x- C nested proxy call and find out the nested dispatch error from the event. Combined with batch call and multi-sig, it just almost impossible to correctly handle all the edge cases.
• Not enough. NonTransfer proxy account can send system.remark, but it doesn't actually means this proxy account have the full ownership.