If a zpool is created by makefs(8), its version is 5000, i.e., all
feature flags are off. Introduce an rc script to run `zpool upgrade`
over the assigned zpools on the first boot. This is useful to the
ZFS based VM images built from release(7).
Reviewed by: imp, markj, mav
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37282