Remailer support

With this feature, uffd can be configured to hide mail addresses of users from certain services while still allowing the services to send mails to the users.

To these services uffd returns special remailer addresses instead of the real mail addresses. When a service sends an email to a remailer address the mail server queries uffd and replaces the remailer address with the real mail address in both envelope and headers.

This feature requires additional mail server configuration (Postfix canonical_maps) and support in uffd-socketmapd.