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