Two new variables: NANO_ROOT and NANO_ALTROOT. These used to be
spelled ${NANO_SLICE_ROOT}a and ${NANO_SLICE_ALTROOT}a respectively,
and that's the default value. This will allow nanobsd on systems
without a bsd label. That's rarely needed these days, even in an MBR
world. The default will shift to this in the future, but remain an
option.