Apply r278265 from head:

Security Advisory information is included on several
pages in the release notes. Rather than duplicating
the information between various files, add two new
files to include in all pages that currently display
the information.

Bump copyright year.

Move SAs and ENs into their respective files.

Add missing ENs and SAs.

Remove stale SAs [1].

Reported by: bdrewery (over a year ago)
Sponsored by: The FreeBSD Foundation