Don't allow OPEN of a PseudoFS file

Also, return a better error for nfs_FhandleToCache failures.

Signed-off-by: Frank S. Filz <ffilz@us.ibm.com>
Fix COMP5 pynfs test

COMP5 sends NFS4_COMPOUND with a variety of invalid operations. Allow these
to be handled by NFS4_OP_ILLEGAL handling.

Particulalry ops 0-2 need to be handled this way.

Signed-off-by: Frank S. Filz <ffilz@us.ibm.com>