allow empty hostname field for several dns record types.

interface/web/dns/form/dns_a.tform.php

interface/web/dns/form/dns_cname.tform.php

interface/web/dns/form/dns_mx.tform.php

interface/web/dns/form/dns_ns.tform.php

interface/web/dns/form/dns_rp.tform.php

interface/web/dns/form/dns_srv.tform.php

interface/web/dns/form/dns_txt.tform.php