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/primary/domains/dodo/aliases

@@ -0,0 +1,5 @@
+# Part 2 of chain-1 (see run.sh for the full structure).
+chain-1-2: chain-1-3@kiwi
+
+# Part 5 of chain-1 (see run.sh for the full structure).
+chain-1-5: user111@dodo