Make Walk spec compliant, mainly to return partial walks

[ramblingenzyme]

the original Walk implementation didn't return partial results, which meant trying to copy a file into a directory caused a Walk failure which aborted the copy command.

The fixes are as follows:

• Rerror is only returned if the first element can't be walked, and partial results get returned otherwise as per spec
• handling .. within Wname, e.g. ["dir", "..", "sibling"]
• add check for the fid being open, newfid already being used

Breaking change:

• WalkFail now only gets called if the dir it would be added to is modifiable. While I can revert this, it would add back in the edge case of the file not being able to be attached to the FS tree.

https://ericvh.github.io/9p-rfc/rfc9p2000.html was where I referenced the spec from, and full disclosure, Claude was used to help me implement this.