FreeBSD boot code reminder after zpool upgrade

There used to be a warning after upgrading a zpool in FreeBSD, so users
won't forget to update the boot loader that pool is booted from.

This change brings this warning back, but only if the bootfs property
is set on the pool, which should be sufficient for the vast majority of
FreeBSD installations. People running something custom are most likely
aware of what to do after an upgrade in their specific environment.

Functionality is implemented in an OS specific helper function.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Michael Gmelin <grembo@FreeBSD.org>
Signed-off-by: Michael Gmelin <grembo@FreeBSD.org>
Closes #12099
Closes #12104