TODO

- rewrite rpc.py code to use some async framework, so we can shut down
  TCP gracefully.
- rewrite rpc.py code to use idl compiler on IDL from RFC1831, instead
  of hand-coded pack/unpack.
- change pyfs docstring to reST, use a Python docstring -> HTML package
  instead of the mk_doc hack.
- Note: many NFS error conditions are caught by client-side code,
  difficult to test.  Write own client code for testing?
- design way to handle multiple filesystems (allocate each its
  own filehandle numbering space, dispatch as needed)
- for rpcgen, think about using metaclass, compile-time code generation, 
  instead of compiling .idl file to .py file, then importing.
- provide general support in srv.py for synthesizing '.' and '..'
  entries, and escaping genuine '.' and '..' entries from filesystems
  (possibly by just appending/deleting '..').
- fix the implementation of directory cookies to be stable across 
  changes to the directory (see srv.py comments).
  This bug will result in, e.g., rm -r failing, because rm -r will
  delete some files, then try to read the rest of a directory, and
  fail because the directory has changed underneath it.