contrib: Add port parsing to makeseeds.py

Allow for non-8333 nodes to appear in the internal seeds. This will
allow bitcoind to bypas a filter on 8333. This also makes it possible to
use the same tool for e.g. testnet.

As hosts with multiple nodes per IP are likely abusive, add a filter to
remove these (the ASN check will take care of them for IPv4, but not
IPv6 or onion).