cr_canseeothergids(9): Revamp, mark as internal

Significantly clarify. Replace references to cr_canseeotheruids(9) by
ones to cr_bsd_visible(9).

Approved by: re (gjb)
Reviewed by: pauamma_gundo.com, mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40633

(cherry picked from commit 3fe9ea4d2d04d48a249b2e6161d416bb4d5b364e)
(cherry picked from commit 15d2ead9623dc2b0a560fc0fd6ba89ca16715597)