rc.d/zpool: change mountcritlocal dep from BEFORE to REQUIRE

In cases where the /boot directory is mounted from a different disk,
/boot/zfs/zpool.cache will not be found during a rc.d/zpool
run. This is because /etc/fstab mounts are mounted in
rc.d/mountcritlocal, which currently runs AFTER (i.e. REQUIRE:)
rc.d/zpool.

This change swaps the rcorder of rc.d/zpool's dependency on
mountcritlocal from BEFORE: to REQUIRE:. This will ensure that
/boot (or even /etc/ in some configurations) to be visible while
searching for zpool.cache.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1614
Closes: https://github.com/freebsd/freebsd-src/pull/1614