SMTP and DNS edits

        Restructured main.cf and master.cf for better reading
        Restructured Postfix section
        Added DANE description for Postfix
        Added DNS as new chapter
        Added DNS /etc/resolv.conf description
        Added DNS resolver explanations (Credit goes to Carsten Strotmann, cs@sys4.de)
Agreed. When we talked about it I was under the impression you wanted me to write about DANE and SMTP. I'm willing to write a more general section about DANE, but I can't before December.