Add support for the "-manage-gids" option to the nfsuserd daemon.
When this option is set, the NFS server uses the list of groups
acquired via getgrouplist(3) for the uid instead of the list of
groups in the RPC request. This can be used to avoid the 16 gid
limit for the group list in the RPC request.

Suggested by: jpaetzel
Tested by: jpaetzel
MFC after: 2 weeks