Implement full support for ZFS -k support. For ZFS, we have to set a
property that gets cleared by the boot loaeder for whether or not to
process nextboot.conf. Do this using system("zfsbootcfg..." rather than
coding the small subset of that program inline to avoid CDDL
contamination of reboot and the complications of disabling CDDL and/or
ZFS. The few bytes needed to implement reboot for systems with zfs is
not worth saving for systems w/o ZFS.

Only set nextboot_enable=YES for UFS filesystems. They are the only one
that need that as the first line. Its presence on ZFS can cause the
kernel to not be oneshot.

Sponsored by: Netflix

This leaves just -o and -e from the nextboot shell script, which isn't as careful as reboot, to be implemented.
I plan on replacing the nextboot.sh script with a symlink and different behavior (not actually rebooting).
It's the lessor of the evils of having our nextboot protocol in multiple places, and continuing support for it in reboot.
The extra code is small (< 2k) and having it all together eventually is good.
Though Kyle did mention a libnextboot, I didn't take that next step because nothing else knows about this protocol from this side of the fence (of course the loader knows...).