postfix: Added experimental support for Berkeley DB (btree files).

Note that hash files are currently not supported by OpenWRT package of Berkeley DB.
CDB is still default.

Signed-off-by: Denis Shulyaka <Shulyaka@gmail.com>

mail/postfix/files/main.cf.default

@@ -24,8 +24,6 @@ address_verify_sender_ttl = 0s
 address_verify_service_name = verify
 address_verify_transport_maps = $transport_maps
 address_verify_virtual_transport = $virtual_transport
-alias_database = cdb:/etc/aliases
-alias_maps = cdb:/etc/aliases
 allow_mail_to_commands = alias, forward
 allow_mail_to_files = alias, forward
 allow_min_user = no
@@ -72,7 +70,6 @@ daemon_timeout = 18000s
 debug_peer_level = 2
 debug_peer_list =
 debugger_command =
-default_database_type = cdb
 default_delivery_slot_cost = 5
 default_delivery_slot_discount = 50
 default_delivery_slot_loan = 3