From: Tim Hockin <thockin@sun.com>,
      Neil Brown <neilb@cse.unsw.edu.au>,
      me

New groups infrastructure.  task->groups and task->ngroups are replaced by
task->group_info.  Group)info is a refcounted, dynamic struct with an array
of pages.  This allows for large numbers of groups.  The current limit of
32 groups has been raised to 64k groups.  It can be raised more by changing
the NGROUPS_MAX constant in limits.h