mountd: prefer non-V4ROOT exports.

If paths A and A/B are both exported, then we have a choice of exports
to return for A (or under A but still above A/B): we could return A
itself, or we could return a V4ROOT export leading to B.

For now, we will always prefer the non-V4ROOT export, whenever that is
an option.  This will allow clients to reach A/B as long as
adminstrators keep to the rule that the security on a parent permits the
union of the access permitted on any descendant.

In the future we may support more complicated arrangements.

(Note: this can't be avoided by simply not creating v4root exports with
the same domain and path, because different domains may have some
overlap.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>