Add options to configure global DNS-Blacklists for postfix
I'm usig following lines in my smtpd_recipient_restrictions directive to control global dnsbls. Please provide a suitable interface to do this as administrator directly in ispconfig. First for comfortable causes and second to have those settings stored even after an upgrade with automatic service reconfiguration.
reject_rbl_client zen.spamhaus.org, reject_rbl_client b.barracudacentral.org, reject_rbl_client cbl.abuseat.org, reject_rbl_client dnsbl.sorbs.net, reject_rbl_client ix.dnsbl.manitu.net, reject_rbl_client combined.njabl.org,