pkg: Add FreeBSD-base to /etc/pkg/FreeBSD.conf

On -CURRENT and -STABLE this gets bits from pkg.freebsd.org; but we get
base system bits from pkgbase.freebsd.org for BETA/RC/RELEASE.

Note that this repository is disabled by default, but can still be used
by explicitly specifying it, e.g. "pkg upgrade -r FreeBSD-base".

With hat: re
MFC after: 8 hours (needed in 15.0-RC1)