afs: Make ONEGROUP_ENV not Linux-specific

afs: Make ONEGROUP_ENV not Linux-specific

The functionality in AFS_LINUX26_ONEGROUP_ENV does not really need to
be Linux-specific (it's just only implemented for Linux right now).
Rename it to AFS_PAG_ONEGROUP_ENV, and remove some Linux-specific
checks when checking for "onegroup" PAG GIDs.

[mmeffie@sinenomine.net: Move AFS_PAG_ONEGROUP_ENV to param.h]

Reviewed-on: https://gerrit.openafs.org/11978
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit ee08dbe37d9db4fe314bd88b9280bf73c92c37bd)

Change-Id: Ifef8f833599eca4241b41035142e74f32e6efa99
Reviewed-on: https://gerrit.openafs.org/12525
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>