Merge branch '6219-duplicate-mail_transport-domain-are-allowed' into develop

install/patches/upd_dev_collection.php

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

interface/lib/classes/validate_mail_transport.inc.php

interface/web/mail/form/mail_transport.tform.php

interface/web/mail/lib/lang/ar_mail_alias.lng

interface/web/mail/lib/lang/ar_mail_forward.lng

interface/web/mail/lib/lang/ar_mail_transport.lng

interface/web/mail/lib/lang/bg_mail_alias.lng

interface/web/mail/lib/lang/bg_mail_forward.lng

interface/web/mail/lib/lang/bg_mail_transport.lng

interface/web/mail/lib/lang/br_mail_transport.lng

interface/web/mail/lib/lang/ca_mail_transport.lng

interface/web/mail/lib/lang/cz_mail_transport.lng

interface/web/mail/lib/lang/de_mail_transport.lng

interface/web/mail/lib/lang/dk_mail_transport.lng

interface/web/mail/lib/lang/el_mail_transport.lng

interface/web/mail/lib/lang/en_mail_alias.lng

interface/web/mail/lib/lang/en_mail_forward.lng

interface/web/mail/lib/lang/en_mail_transport.lng