namecache coherency 3rd turn

[Csaba Henk]

Minor update:

 - made cache code topology agnostic (ie., tree layout hints [height] are
   maintained but not used by the cache code itself, group members are
   treated uniformly);

 - made group traversal routine public;

 - panic fix for nullfs.

The respective diffs for the cache system and nullfs are available here:

  http://creo.hu/~csaba/tmp/visible/dfly/

as well as incremental diffs from the latest posted patches.

Regards,
Csaba