A future change may split cr_gid out of cr_groups[0] so that there's a cleaner separation between the supplemental groups and the effective group. Do the mechanical conversion where we can, and drop some comments where we need further work because some assumptions about cr_gid == cr_groups[0] have been made. This should not be a functional change, but downstreams and other out-of-tree code are advised to investigate their usage of cr_groups sooner rather than later, as a future change will render assumptions about these two being equivalent harmful. Reviewed by: asomers, kib, olce Differential Revision: https://reviews.freebsd.org/D51153