Refactor the root mount hold code and add the wait to etc/rc.d/fsck.
This fixes mounting (non-root) USB drives on boot with fsck enabled
(with non-zero 'Pass#' field in fstab(5)).

Reported by: Graham Menhennitt <graham at menhennitt.com.au>
Reviewed by: jilles@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6221