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/.gitignore

@@ -0,0 +1,3 @@
+primary/**/users
+secondary/**/users
+external/**/users