More robust handling of whois referrals from RIRs.

An example problem case is 163.1.0.0 (University of Oxford)
which is in an APNIC ERX address range. Previously we assumed
that ARIN has the correct information for all ERX allocations,
but in this case ARIN refers back to APNIC, rather than referring
to RIPE. This caused whois to loop.

Whois will no longer loop back and forth forever between two RIRs
that don't have an answer, but instead try the other RIRs in turn.