refactor: Move port mapping code to its own module

This commit does not change behavior.

src/node/interfaces.cpp

@@ -12,6 +12,7 @@
 #include <interfaces/handler.h>
 #include <interfaces/node.h>
 #include <interfaces/wallet.h>
+#include <mapport.h>
 #include <net.h>
 #include <net_processing.h>
 #include <netaddress.h>