Allow use of opened fid for walking
Allow use of opened fid for walking

Walk(5) explicitly states

          The fid must be valid in the current session and must not
          have been opened for I/O by an open or create message.

However, I find that `9pfuse` does not respect this, and will happily
use an opened fid for walking. Because `9pfuse` is a useful tool
for testing my implementation, I will allow this behavior. However,
I will see what the community has to say about this mismatch
in docs vs reality.

droyo/styx