Add access lookup tables for Postfix

[martbock]

Postfix uses lookup tables for access restrictions. These lookup tables can be Berkeley DBs - but it should also be possible to use an SQL query. This is where MUM comes into play. I want MUM to have a web interface for the super admin, where all types of access restrictions may be configured. We would also need to write SQL queries for Postfix to use our restrictions.

It should be possible to include (at least) the following access restrictions:

• check_recipient_access
• check_client_access
• check_helo_access
• check_sender_access
• postscreen_access_list

There are many more access checks we could implement like check_ccert_access, check_client_a_access, check_client_mx_access, check_client_ns_access, check_reverse_client_hostname_access, check_reverse_client_hostname_a_access, check_reverse_client_hostname_mx_access, check_reverse_client_hostname_ns_access or check_sasl_access - but I don't know if these checks would be particularly useful.