pts mem -supergroup option

pts mem -supergroup option

Improve pts support for supergroups with an option to list the
supergroups of a group.

Change-Id: I4fe1cd131cd334386bc16ce733e01e29e0511d4f
Reviewed-on: http://gerrit.openafs.org/1600
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>