how styx operations are handled ("converted to sftp messages").  and which responses we can get.

Version
	nothing special
Auth
	no auth required
Attach
	return a made-up root
Flush
	wait until sftp sequence is done before responding.  when we get a flush, we see if the styx tag is active.  if not, we respond immediately.  when an sftp sequence is done, and we respond to the styx message, we check (recursively, for flushes too!) whether the tag is being flushed, and we respond with the flushes too.
Walk
	Tmsg.Stat -> Tsftp.Stat -> Rsftp.Attrs -> Rmsg.Walk
	                        -> Rsftp.Error -> Rmsg.Error
	for cloning the fid, nothing special is required.
	a walk with non-empty list of names does a Stat.  the responses:
	Attr, on succes
	Status, on error
Open
	Tmsg.Open -> Tsftp.Opendir/Tsftp.Open -> Rsftp.Handle -> Rmsg.Open
	                                      -> Rsftp.Status -> Rmsg.Error
	if isdir, do sftp opendir
	else, do sftp open
	sftp responses are the same in both cases:
	Handle, on success
	Status, on error

Create
	if we have to create a directory:
		issue sftp mkdir
		responses:
		Status (both on failure & success)
		if success, we have to open the directory:
			send sftpopendir
			responses (just like in Open):
			Handle, on success
			Status, on error
	else (create a normal file)
		send sftp open
		responses:
		Handle, on success
		Status, on error
Read
	if is directory:
		send sftp readdir message
		responses:
		Status, on error
		Status (EOF), on eof
		Name, on success & not yet eof
	else (not a directory):
		send an sftp read
		responses:
		Status, on error
		Status (EOF), on eof
		Data, on success & not yet eof
Write
	send sftp write
	responses:
	Status (both on success and on error)
Clunk
	for open file, have to issue an sftp close
	responses:
	Status (both on success & error, but we'll ignore it)
Stat
	send sftp stat.
	responses:
	Status, on error
	Attr, on success
Remove
	if the file was open, we first have to send an sftp close message.
	we continue.
	if the file to remove is a directory:
		send an sftp rmdir
		responses:
		Status (both on success and error)
	else (not a directory)
		send an sftp remove
		responses:
		Status (both on success and error)
Wstat
	send an sftp stat.
	responses:
	Status, on error
	Attr, on success
	then, we have to change the resulting attr with the Dir from the wstat request.
	we send the new attrs in an sftp setstat
	responses:
	Status (both on success and error)
	then, we might have to set the name (setstat doesn't do that for us):
	send sftp rename
	responses:
	Status (both for success & error)
