pimd: temporarily disable IPv6 types

pimd: temporarily disable IPv6 types

Just putting the infrastructure in place and having it disabled is
actually good progress here;  have the compiler make itself useful and
tell us what we have to do to get the basics right.

(The next commit will cause a *lot* of compile errors as soon as
`PIM_V6_TEMP_BREAK` is set; but there is no reason to force everything
into a single step here.)

To enable `pim_addr = in6_addr`, run `make PIM_V6_TEMP_BREAK=1` (remove
previous compile results with `rm pimd/pim6d-*.o`)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>