Make ktc_curpag also detect ONEGROUP PAG gids

Make ktc_curpag also detect ONEGROUP PAG gids

ktc_curpag falls back to looking at the group list if the VIOC_GETPAG
pioctl fails. If we're in AFS_LINUX26_ONEGROUP_ENV in the kernel,
though, ktc_curpag still looks for two groups, instead of the one
combined group. Add a check for the big one group in the fallback if
we're on LINUX26.

Change-Id: I28e5eda5c62f13a6fb466c8a2b04d2628706498f
Reviewed-on: http://gerrit.openafs.org/815
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>