Treat D keywords as identifiers in certain postfix expressions. This allows
one to, for example, access the "provider" field of a struct g_consumer,
even though "provider" is a D keyword.

PR:		169657
MFC after:	2 months
Discussed with:	Bryan Cantrill
Sponsored by:	EMC / Isilon Storage Division
Add explicit_bzero(3) and its kernel counterpart.

Obtained from:	OpenBSD
MFC after:	2 weeks
Bump __FreeBSD_version for the addition of explicit_bzero(3).
Add MLINK for explicit_bzero(3) and bump .Dd date.

MFC after:	2 weeks