Linux 4.9: deal with demise of GROUP_AT

Linux 4.9: deal with demise of GROUP_AT

Linux commit 81243eacfa40 "cred: simpler, 1D supplementary groups"
refactors the group_info struct, removing some members (which OpenAFS
references only through the GROUP_AT macro) and adding a gid member.
The GROUP_AT macro is also removed from the tree.

Add an autoconfigure test for the new group_info member gid and define a
replacement GROUP_AT macro to do the right thing under the new regime.

Change-Id: I85a52c0ae0d91fc141a523f443a4ffc05eb72a2b
Reviewed-on: https://gerrit.openafs.org/12390
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>