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".

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

(cherry picked from commit 8e0b1a1c03a35a5db13e370cb3e62585d3db68e3)
(cherry picked from commit 91e1c18012dc839f400f971f02bc08798c6b9ce9)