rc(8): Add a zpoolupgrade rc.d script

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