rc.d/mountcritlocal: Make sure zpools are imported for legacy ZFS

Legacy ZFS uses fstab to mount its datasets. In an attempt to fix
another problem 900bc0206348 broke legacy ZFS in fstab(5). This
comit works around the problem by mountcritlocal scanning /etc/fstab
for zfs mountpoint and if any are found invoke /etc/rc.d/zpool start.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50844