net: Add -networkactive option

The `setnetworkactive' RPC command is already present.
This new option allows to start the client with disabled p2p network
activity for testing or reindexing.

src/net.h

@@ -181,7 +181,7 @@ public:
         }
     }
 
-    CConnman(uint64_t seed0, uint64_t seed1);
+    CConnman(uint64_t seed0, uint64_t seed1, bool network_active = true);
     ~CConnman();
     bool Start(CScheduler& scheduler, const Options& options);