Move NatController startup to iptables-restore.

This makes NatController startup much faster. Before:
02-03 14:09:19.199   485   485 I Netd    : Setting up NatController hooks: 223.4ms

After:
02-03 14:28:09.407   488   488 I Netd    : Setting up NatController hooks: 13.8ms

Also, fail if MSS rewriting is not supported. It's much easier
to diagnose a failure than a performance problem.

Test: unit tests continue to pass
Test: bullhead builds, boots, wifi tethering works
Fix: 17629786
Bug: 34873832
Change-Id: I0ef498ecbbb82a2672c279056d189d026e13100a
diff --git a/server/NatController.h b/server/NatController.h
index 939f09d..5541a26 100644
--- a/server/NatController.h
+++ b/server/NatController.h
@@ -21,6 +21,8 @@
 #include <list>
 #include <string>
 
+#include "NetdConstants.h"
+
 class NatController {
 public:
     NatController();
@@ -52,6 +54,7 @@
     // For testing.
     friend class NatControllerTest;
     static int (*execFunction)(int, char **, int *, bool, bool);
+    static int (*iptablesRestoreFunction)(IptablesTarget, const std::string&);
 };
 
 #endif