Make cr_bsd_visible()'s sub-functions internal

cr_canseeotheruids(), cr_canseeothergids() and cr_canseejailproc()
should not be used directly now. cr_bsd_visible() has to be called
instead.

Reviewed by: mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40629

(cherry picked from commit 91e9d669b475d1900e8dc01a49ad90a621c4a068)