Teach /etc/rc.d/growfs how to handle systems running ZFS.

There are many cases which this code does not handle (e.g. ZFS mirrors)
but the code can handle the single-disk case -- so it's enough to take
care of the "disk image which gets booted into a VM with a larger than
expected disk" case for which this firstboot script was created.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19095