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/secondary/domains/dodo/aliases

@@ -0,0 +1,8 @@
+# Part 1 of chain-1 (see run.sh for the full structure).
+chain-1-1: chain-1-2@dodo via primary
+
+# Part 4 chain-1 (see run.sh for the full structure).
+chain-1-4: chain-1-5@dodo via primary
+
+# Forward all email to the primary server.
+*: * via primary