Add the ldap/aliases map to virtual_alias_maps.

Just before the delivery to the destination mailbox.

Part of issue #172, parameterize the Postfix alias/transport maps.

Move the infrastructure code to the 'float' subdirectory, preparing
for splitting it into a separate repository, but keep the ai-specific
stuff in the 'rules' directory as before.

Also add config for the postfix-delivery instance.

Start adding LDAP support, and complete the frontend mail routing.