reboot: Implement zfs support

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
Reviewed by: kevans, kib
Differential Revision: https://reviews.freebsd.org/D43824