Don't create debug.log immediately upon startup

Instead of creating the debug.log immediately when bro starts,
now it is created only after the debug streams are enabled.
This avoids having an empty log being created when it shouldn't be,
in usages such as "bro -h", "bro -v", or "bro -B help" (and also
when using broctl, which needs to run "bro -v").

src/main.cc

@@ -755,6 +755,8 @@ int main(int argc, char** argv)
 #ifdef DEBUG
 	if ( debug_streams )
 		debug_logger.EnableStreams(debug_streams);
+
+	debug_logger.OpenDebugLog("debug");
 #endif
 
 	init_random_seed(seed, (seed_load_file && *seed_load_file ? seed_load_file : 0) , seed_save_file);