Remove all traces of logfile_t

The earlier log file based approach for enabling and disabling
messages has now been completely replaced with the syslog priority
based approach.

Similarly as with log files before it is now possible to enable
and disable a log priority for a particular session, even though
it apparently has not been used much.

The local test-programs of the logging has got minimal attention
only to make them compile. They should get an overhaul as they did
not work before either.

server/modules/protocol/mysql_client.c

@@ -487,7 +487,7 @@ static int gw_mysql_do_authentication(DCB *dcb, GWBUF **buf) {
 		return MYSQL_FAILED_AUTH_SSL;
 	    }
 
-	    if(LOG_IS_ENABLED(LT) && ssl)
+	    if (MXS_LOG_PRIORITY_IS_ENABLED(LOG_INFO) && ssl)
 	    {
 		MXS_INFO("User %s@%s connected to service '%s' with SSL.",
 			 protocol->owner_dcb->user,