rc: Add a zpoolreguid rc.d script

If one boots up multiple copies of a template VM image containing a
zpool, the pool GUIDs will be identical, making it impossible to, e.g.,
share datasets between them.

This diff introduces a simple workaround for the problem: one can use
the script to, upon first boot, assign a new GUID to one or more zpools.
This will be useful when building ZFS-based VM images from release(7).

Reviewed by: mav, allanjude, asomers
Reviewed by: Pau Amma (docs)
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35336