Improve OpenRC init script

The current zfs OpenRC script's dependencies cause OpenRC to attempt to
unmount ZFS filesystems at shutdown while things were still using them,
which would fail. This is a cosmetic issue, but it should still be
addressed. It probably does not affect systems where the rootfs is a
legacy filesystem, but any system with the rootfs on ZFS needs to run
the ZFS init script after the system is ready to shutdown filesystems.

OpenRC's shutdown process occurs in the reverse order of the startup
process. Therefore running the ZFS shutdown procedure after filesystems
are ready to be unmounted requires running the startup procedure before
fstab. This patch changes the dependencies of the script to expliclty
run before fstab at boot when the rootfs is ZFS and to run after fstab
at boot whenever the rootfs is not ZFS. This should cover most use
cases.

The only cases not covered well by this are systems with legacy
root filesystems where people want to configure fstab to mount a non-ZFS
filesystem off a zvol and possibly also systems whose pools are stored
on network block devices. The former requires that the ZFS script run
before fstab, which could cause ZFS datasets to mount too early and
appear under the fstab mount points. The latter requires that the ZFS
script run after networking starts, which precludes the ability to store
any system information on ZFS. An additional OpenRC script could be
written to handle non-root pools on network block devices, but that will
depend on user demand and developer time.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1479