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)
Of course you can add more than three nameservers, but for historical reasons MTAs will ignore a fourth nameserver and any subsequent ones.