Shut mitmdump down gracefully on SIGTERM.

This is a hack at the moment, but needs must.

libmproxy/dump.py

@@ -214,6 +214,11 @@ class DumpMaster(flow.FlowMaster):
             self._process_flow(f)
         return f
 
+    def shutdown(self):
+        if self.o.wfile:
+            self.wfile.fo.close()
+        return flow.FlowMaster.shutdown(self)
+
 # begin nocover
     def run(self):
         if self.o.rfile and not self.o.keepserving: