[Qt] allow setting listen via GUI

- add DEFAULT_LISTEN in net.h and use in the code (shared
  setting between core and GUI)

Important: This makes it obvious, that we need to re-think the
settings/options handling, as GUI settings are processed before
any parameter-interaction (which is mostly important for network
stuff) in AppInit2()!

src/init.cpp

@@ -766,7 +766,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     }
 
     // see Step 2: parameter interactions for more information about these
-    fListen = GetBoolArg("-listen", true);
+    fListen = GetBoolArg("-listen", DEFAULT_LISTEN);
     fDiscover = GetBoolArg("-discover", true);
     fNameLookup = GetBoolArg("-dns", true);