p_cansee(9): Bring up-to-date, misc fixes

Essentially defer to cr_cansee(9), except for the specifics.

Be more specific on the return codes.

Approved by: re (gjb)
Reviewed by: bcr, pauamma_gundo.com
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40637

(cherry picked from commit 2ede38aff5d4c91a17ab6d093f2e8cce24b5418b)
(cherry picked from commit 57d0cc50cd2878fcca8a9a4bf931abee0260b48d)