Add K_NUL definition.

exec/load/sbbsdefs.js

@@ -204,6 +204,7 @@ var   K_NOSPIN      =(1<<21);	/* Do not honor user's spinning cursor		*/
 var   K_ANSI_CPR	=(1<<22);	/* ANSI Cursor Position Report expected		*/
 var   K_TRIM        =(1<<23);   /* Trim white-space from both ends of str   */
 var   K_CTRLKEYS	=(1<<24);	/* No control-key handling/eating in inkey()*/
+var   K_NUL         =(1<<25);   /* Return null instead of "" upon timeout   */
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