poll both stdin and mpd using one select()

this allows for immediate reading mpd events

src/status.h

@@ -44,6 +44,7 @@ Window &Statusbar();
 void DrawProgressbar(unsigned elapsed, unsigned time);
 void ShowMessage(const char *, ...) GNUC_PRINTF(1, 2);
 
+void StatusbarMPDCallback();
 void StatusbarGetStringHelper(const std::wstring &);
 void StatusbarApplyFilterImmediately(const std::wstring &);