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's API 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.