vfsops - add two new ops, ncpgen_set, ncpgen_test

vfsops - add two new ops, ncpgen_set, ncpgen_test

 * ncpgen_set takes care of setting the namecache generation in an ncp,
   while ncpgen_test can force zapping a resolved negative cache hit.

 * Make use of the functions in devfs to keep the functionality we had
   before.

 * Also pass the calls through in nullfs - this should fix a problem
   with posix_openpt when /dev is a nullfs mount of devfs.

Reported-by: Markus Pfeiffer (profmakx)