nfsd: new interface to advertise export features

Soon we will add the new V4ROOT flag, and allow the INSECURE flag to
vary by pseudoflavor.  It would be useful for nfs-utils (for example,
for improved exportfs error reporting) to be able to know when this
happens.  Use this new interface for that purpose.

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