pkgbase: Create a FreeBSD-fetch package

It's useful for small image to fetch some data but we don't want to
install utilities nor bloat runtime.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33463

(cherry picked from commit 13ef8134efd1c0a39c2dab0197b5c4558101253e)