exim: Completely review the exim configuration.

This commit mostly removes the fancy Debian debconf automatic stuff,
since that creates a lot of overhead with macros being defined in one
place and the actual configuration in another place. Other unused parts
of the configuration are also unused.

Configuration is added for the main delivery lookups to happen from
ldap data. Both persons and groups in the LDAP directory can have email
addresses defined, which will then get forwarded or delivered
appropriately (for emailaddresses in the virtual_domains setting, of
course). Email will also be delivered for any username@host addresses,
where host is one of the drsnuggles vservers.