Fixed: FS#3336 - Underscore shouln't be possible in DNS entries!

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

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

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

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

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

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

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

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

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

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

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

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

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