[Minor] Align scores of Spamhaus DBL, SURBL, URIBL DNSBL symbols

[Minor] Align scores of Spamhaus DBL, SURBL, URIBL DNSBL symbols

Given that they have about the same false positive rate, it makes sense
to treat them equal in terms of scoring:
- Particular threats (phishing, malware) are scored a bit higher than
  mere spam domain listings
- "Abused legitimate" listings are scored lower for some DNSBLs already,
  this has now been aligned.
- For SURBL, cracked and abused sites are treated with the same score.