Add Sequence number to OpError type

[oskirby]

I am trying to write an asynchronous wrapper around a netlink.Conn so that a single connection can be shared amongst many goroutines and it's been working pretty good so far, except when it comes to handling errors. The problem arises because the OpError type doesn't include anything that can be used to match up with the request that generated the error. It would be handy if we could include the Sequence field from the netlink header in the OpError to facilitate such a match.