9P: fixes a regression observed after 9P passed under control of the export manager.
The bug was this : export was "put" each time a fid is clunked. But many fid are generated and clunked
in the life of a 9P server. The only fid that need to have the related export put as it is clunked is the fid obtained by
a call to TATTACH or TAUTH. A flag was added to _9p_fid_t to detect this special fid.