Remailer support

Adds "remailer_canonical" request that allows canonical_map address rewriting with Postfix for uffd's remailer feature.

README.md

@@ -7,7 +7,7 @@ for a given address. uffd-socketmapd uses this API to integrate alias lookup
 with MTAs that support the socketmap protocol, like sendmail and postfix.
 
 uffd-socketmapd can be run manually. For production deployments, use the
-provided debian packages. Add our package mirror to `/etc/sources.list`:
+provided Debian packages. Add our package mirror to `/etc/sources.list`:
 
 ```
 deb https://packages.cccv.de/uffd bullseye main
@@ -26,6 +26,14 @@ by adding the following lines to `/etc/postfix/main.cf`:
 virtual_alias_maps = socketmap:unix:/uffd-socketmapd.sock:virtual
 # Defaults to $virtual_alias_maps, which does not work here, so unset it
 virtual_alias_domains =
+
+# Optional: To use uffd's remailer feature, setup address rewriting. Remailer
+# recipient addresses will be rewritten in both the envelope (like virtual
+# aliases) and the message headers. Make sure that rewriting takes place
+# before DKIM signing and that it is only applied to messages from your
+# services (see local_header_rewrite_clients).
+recipient_canonical_maps = socketmap:unix:/uffd-socketmapd.sock:remailer_canonical
+local_header_rewrite_clients = permit_inet_interfaces permit_sasl_authenticated
 ```
 
 Note that uffd-socketmapd requires at least uffd v1.2.0!