add FWalk1Op and Clone interfaces for srv

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from 9p(2):

Because implementing the full walk message is intricate
               and prone to error, the helper routine walkandclone
               will handle the request given pointers to two functions
               walk1 and (optionally) clone . Clone, if non-nil, is
               called to signal the creation of newfid from oldfid.
               Typically a clone routine will copy or increment a ref-
               erence count in oldfid's aux element.  Walk1 should
               walk fid to name, initializing fid->qid to the new
               path's qid.  Both should return nil on success or an
               error message on error.  Walkandclone will call respond
               after handling the request.

Original issue reported on code.google.com by mischief@offblast.org on 10 Mar 2014 at 12:36