Zmailer RBL support

[Arnt Gulbrandsen <agulbra@troll.no>]

I hacked RBL support into zmailer.  RBL is a Realtime Blocking List -
a nice database of people one really should not accept SMTP from.  See
http://maps.vix.com for info, here's just an example of its contents:

    $ dig 50.212.199.205.rbl.maps.vix.com any

    ; <<>> DiG 8.1 <<>> 50.212.199.205.rbl.maps.vix.com any
    ;; res options: init recurs defnam dnsrch
    ;; got answer:
    ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6
    ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 5, ADDITIONAL: 6
    ;; QUERY SECTION:
    ;;      50.212.199.205.rbl.maps.vix.com, type = ANY, class = IN

    ;; ANSWER SECTION:
    50.212.199.205.rbl.maps.vix.com.  5M IN A  127.0.0.2
    50.212.199.205.rbl.maps.vix.com.  5M IN TXT  "Blackholed - see <URL:http://maps.vix.com/cgi-bin/lookup?205.199.212.0>"

I'm not terribly proud of this hack, it assumes that zmailer talks
ipv4 and not ipv6, and further that mail from blackholed sites is to
be 5xx'ed, not 4xx'ed.  Not as flexible as Matti's spam code, but I
didn't see it could be integrated neatly into policytest.c.

But if anyone wants the patch for smtpserver, write me.

--Arnt