mapport: rename 'use_pcp' to 'enable'

There is only a single protocol now, caller should just be concerned about whether to enable port mapping or not.

src/node/interfaces.cpp

@@ -187,7 +187,7 @@ public:
         });
         args().WriteSettingsFile();
     }
-    void mapPort(bool use_pcp) override { StartMapPort(use_pcp); }
+    void mapPort(bool enable) override { StartMapPort(enable); }
     bool getProxy(Network net, Proxy& proxy_info) override { return GetProxy(net, proxy_info); }
     size_t getNodeCount(ConnectionDirection flags) override
     {