Only emit network messages once we have seen two of them

(except in --net-verbatim mode, where we emit them all)

Move aircraft tracking into track.[ch].

Clean up references to "interactive mode" when tracking
aircraft - we always track aircraft, even in non-interactive
mode.

util.h

@@ -0,0 +1,27 @@
+// Part of dump1090, a Mode S message decoder for RTLSDR devices.
+//
+// track.h: aircraft state tracking prototypes
+//
+// Copyright (c) 2015 Oliver Jowett <oliver@mutability.co.uk>
+//
+// This file is free software: you may copy, redistribute and/or modify it  
+// under the terms of the GNU General Public License as published by the
+// Free Software Foundation, either version 2 of the License, or (at your  
+// option) any later version.  
+//
+// This file is distributed in the hope that it will be useful, but  
+// WITHOUT ANY WARRANTY; without even the implied warranty of  
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License  
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef DUMP1090_UTIL_H
+#define DUMP1090_UTIL_H
+
+#include <stdint.h>
+
+uint64_t mstime(void);
+
+#endif