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/interfaces/node.h

@@ -121,7 +121,7 @@ public:
     virtual void resetSettings() = 0;
 
     //! Map port.
-    virtual void mapPort(bool use_pcp) = 0;
+    virtual void mapPort(bool enable) = 0;
 
     //! Get proxy.
     virtual bool getProxy(Network net, Proxy& proxy_info) = 0;