cr_canseeotheruids(9): Revamp, mark as internal

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

Approved by: re (gjb)
Reviewed by: bcr, mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40635

(cherry picked from commit 4ddd253b38dff8725555355cc1b5238b1bbfd380)
(cherry picked from commit d05ed9a37b84f277cbc06447b322930fe678b00e)