kern: adopt the cr_gid macro for cr_groups[0] more widely

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