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/mapport.cpp

@@ -166,9 +166,9 @@ static void MapPortProtoSetEnabled(bool enabled)
     g_mapport_enabled = enabled;
 }
 
-void StartMapPort(bool use_pcp)
+void StartMapPort(bool enable)
 {
-    MapPortProtoSetEnabled(use_pcp);
+    MapPortProtoSetEnabled(enable);
     DispatchMapPort();
 }