pt: let pt_util create the groups it wants

if you are rebuilding from pt_util, data sanitization should
not randomly chown and/or rename your groups. likewise,
an admin should have the ability to do this.

Change-Id: Id76cd16acc4af712b9be6d0c7110e8db10ce3fff
Reviewed-on: http://gerrit.openafs.org/9841
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
dasalvager: use pthreads

salvageserver already uses this code, and it appears to do the
right thing.

Change-Id: I4c0f2ec503f2388a1483e89a04ff78eda270659b
Reviewed-on: http://gerrit.openafs.org/9843
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>