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.

Reviewed by: bcr, pauamma_gundo.com
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40637