start marking and using fid types

Each dispatch function must now explicitly call for the
types of fids it allows (or rejects).  This lets us make
sure insane clients will not send xattr fids into walk
and mkdir operations, for instance, which would confuse
the back-end code.

(This also fixes existing issues with broken clients doing
things like using Twalk on an open directory, which can leak
open file descriptors.  The protocol prohibits this but the
existing backend code never checks.)

This has no effect on any well-behaved client, since the
backend has no extattr code yet, but it is needed to enable
the backend to work with attribute fids, which need special
action on clunk.  Rather than making the backend check,
we provide separate xattr read, write, and clunk here.

backend/backend.h

@@ -53,6 +53,9 @@ struct l9p_backend {
 	int (*setattr)(void *, struct l9p_request *);
 	int (*xattrwalk)(void *, struct l9p_request *);
 	int (*xattrcreate)(void *, struct l9p_request *);
+	int (*xattrread)(void *, struct l9p_request *);
+	int (*xattrwrite)(void *, struct l9p_request *);
+	int (*xattrclunk)(void *, struct l9p_fid *);
 	int (*readdir)(void *, struct l9p_request *);
 	int (*fsync)(void *, struct l9p_request *);
 	int (*lock)(void *, struct l9p_request *);