Add flags to selectp to enable and disable input to a pane, from Anish

Athalye.

tmux.h

@@ -956,6 +956,7 @@ struct window_pane {
 #define PANE_FOCUSED 0x4
 #define PANE_RESIZE 0x8
 #define PANE_FOCUSPUSH 0x10
+#define PANE_INPUTOFF 0x20
 
 	int		 argc;
 	char	       **argv;