Add explicit_bzero(3) and its kernel counterpart.

Obtained from:	OpenBSD
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272673 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Bump __FreeBSD_version for the addition of explicit_bzero(3).


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272674 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add MLINK for explicit_bzero(3) and bump .Dd date.

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272675 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f