aliases: Implement "via" aliases

This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.

This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.

For now, it is experimental and the syntax and semantics are subject to
change.

test/t-22-forward_via/smtpc-secondary.conf

@@ -0,0 +1,4 @@
+addr localhost:2465
+server_cert secondary/certs/secondary/fullchain.pem
+user user222@dodo
+password user222