Make NATing add/remove iptable rules as needed

It was flushing on every unNAT, but really you want to remove just those rules.  We'll
Flush when we get to 0 NATs.

bug: 2542176
Change-Id: Ia70580191b1aed754689864044de122234346011
diff --git a/NatController.h b/NatController.h
index 4182f1d..1ab0202 100644
--- a/NatController.h
+++ b/NatController.h
@@ -31,9 +31,12 @@
     int disableNat(const char *intIface, const char *extIface);
 
 private:
+    int natCount;
+
     int setDefaults();
     int runIptablesCmd(const char *cmd);
     bool interfaceExists(const char *iface);
+    int doNatCommands(const char *intIface, const char *extIface, bool add);
 };
 
 #endif