Improved the DNS manager.

install/sql/ispconfig3.sql

interface/web/dns/dns_a_edit.php

interface/web/dns/dns_alias_edit.php

interface/web/dns/dns_cname_edit.php

interface/web/dns/dns_hinfo_edit.php

interface/web/dns/dns_mx_edit.php

interface/web/dns/dns_ns_edit.php

interface/web/dns/dns_ptr_edit.php

interface/web/dns/dns_rp_edit.php

interface/web/dns/dns_srv_edit.php

interface/web/dns/dns_txt_edit.php