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]

Change-Id: I01d29fff309337ae95b9b6c65db3d2212cf4bf89
Reviewed-on: https://gerrit.openafs.org/11978
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>