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)
it's good that DANE is mentioned, but in my opinion this should be more general as DANE might not only be used for mail transport via SMTP.