Keep two versions of the FreeBSD.conf pkg configuration file; one which
points at the "latest" branch and one which points at the "quarterly"
branch. Install the "latest" version unless overridden via the newly
added PKGCONFBRANCH variable.
This does not change user-visible behaviour (assuming said vairable is
not set) but will make it easier to change the defaults in the future --
on stable branches we will want "latest" on x86 but "quarterly" elsewhere.
Discussed with: gjb
MFC after: 3 days
X-MFC: After MFCing this I'll make a direct commit to stable/* to
switch non-x86 architectures to "quarterly".