The previous implementation assumed that you can't attach more than once. This is clearly wrong, Tattach and Twalk are the two ways to obtain new fids. This drops the error on subsequential attach and making the test "multiple attach" passes.

at the start of the loop nwqid is 0, so if the first component can't be opened nwqid becomes -1 and since it's not 0, we end up calling np_walk with -1 as length. This in turns converts it back to uint16_t and we generate an invalid packet. The solution is to not decrement nwqid at all, it fixes all the current tests case and is the correct behaviour that the rest of the code expects.

disallow empty path, the dot or components which contains the path separator ('/'). The current implementation transforms these into a "can't open" type of failure, I'm unsure if we want to turn these into hard Rerror.

spotted by cage, thanks :)