Added wxFDIOHandler::IsOk() and use it with wxSocketImplUnix.

This will allow to use the base wxFDIOHandler class only in GUI-specific
network code and this remove its dependency on wxSocketImplUnix. IOW it paves
the way for a proper solution of the problem fixed by r61336 without the hack
of r61335 which results in linking problems (which went undiscovered until now
but were, in fact, always present, i.e. r61335 couldn't work).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61685 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk1/sockgtk.cpp

@@ -37,7 +37,7 @@ void wxSocket_GDK_Input(gpointer data,
         // we could have lost connection while reading in which case we
         // shouldn't call OnWriteWaiting() as the socket is now closed and it
         // would assert
-        if ( handler->m_fd == INVALID_SOCKET )
+        if ( !handler->IsOk() )
             return;
     }