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/external/domains/kiwi/aliases

@@ -0,0 +1,2 @@
+# Part 3 of chain-1 (see run.sh for the full structure).
+chain-1-3: chain-1-4@dodo via secondary