Add new Tunnel::delay_teredo_confirmation option, default to true.

This option indicates that the Teredo analyzer should wait until
it sees both sides of a connection using a valid Teredo encapsulation
before issuing a protocol_confirmation.  Previous behavior confirmed
on the first instance of a valid encapsulation, which could result
in more false positives (and e.g. bogus entries in known-services.log).

Addresses #890.

src/const.bif

@@ -16,6 +16,7 @@ const Tunnel::enable_ip: bool;
 const Tunnel::enable_ayiya: bool;
 const Tunnel::enable_teredo: bool;
 const Tunnel::yielding_teredo_decapsulation: bool;
+const Tunnel::delay_teredo_confirmation: bool;
 const Tunnel::ip_tunnel_timeout: interval;
 
 const Threading::heartbeat_interval: interval;