special cases for it. This is to simplify the code to prepare it
for pullup to netbsd-4 and submitting back upstream. The change
was requested by martin@.
allowing rules to be set to match only ipv4/ipv6. And so ipnat must be updated
to actually set this field correctly but to keep things working for old
versions of ipnat (that will set this to 0), make the ioctl handler "update"
the 0 to a 4 to keep things working when people just upgrade kernels. This
forces NAT rule matching to be limited to ipv4 only, here forward, fixing
kern/28662
