zoneconf.in (local-addresses-p): Set fake addresses in env var.

It's useful to be able to test configurations away from the production
environment, but the `local-addresses-p' check causes the results to be
different.  To allow this kind of testing, set the environment variable
`ZONECONF_LOCAL_ADDRESSES' to a `:'-separated list of IP addresses, in
dotted-quad format: then these addresses and no others will be
considered local.