Add support to Bro for connecting with peers over IPv6.

- Communication::listen_ipv6 needs to be redef'd to true in order
  for IPv6 listening sockets to be opened.

- Added Communication::listen_retry option as an interval at which
  to retry binding to socket addresses that were already in use.

- Added some explicit baselines to check in the istate.events
  and istate.events-ssl tests -- the SSL test was incorrectly
  passing because it compared two empty files.  (The files being
  empty because "http/base" was given as an argument to Bro which
  it couldn't handle because that script doesn't exist anymore).

scripts/policy/frameworks/communication/listen.bro

@@ -8,5 +8,6 @@ module Communication;
 event bro_init() &priority=-10
 	{
 	enable_communication();
-	listen(listen_interface, listen_port, listen_ssl);
+	listen(listen_interface, listen_port, listen_ssl, listen_ipv6,
+	       listen_retry);
 	}