Add support for three boolean options: krb5kdcdns, krb5realmdns, preferdns

Add three boolean options which configure use of DNS in kerberos. These all default to `false` so there will be no change for existing users who upgrade - users must opt-in by setting these booleans to `true`.
Why is `$preferedns_flg` in the `$krb5_flags` ? The description of the params is as follow

    --enablepreferdns to prefer DNS over NIS or WINS in hostname resolution

I am not sure its a logical grouping or I might be missing somethings. Let's create an `$extra_flags` section, so grouping remains logic if it applies.