Fix recent changes related to USABLE_SIGIO

* src/process.c (wait_reading_process_output) [WINDOWSNT]:
* src/keyboard.c (handle_async_input) [DOS_NT]: Ifdef away the
code that is not needed on MS-Windows.  (Bug#50403) (Bug#51820)

src/keyboard.c

@@ -7180,6 +7180,7 @@ tty_read_avail_input (struct terminal *terminal,
 static void
 handle_async_input (void)
 {
+#ifndef DOS_NT
   while (1)
     {
       int nread = gobble_input ();
@@ -7189,6 +7190,7 @@ handle_async_input (void)
       if (nread <= 0)
 	break;
     }
+#endif
 }
 
 void

src/process.c

@@ -5588,7 +5588,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 	    timeout = make_timespec (0, 0);
 #endif
 
-#ifndef USABLE_SIGIO
+#if !defined USABLE_SIGIO && !defined WINDOWSNT
 	  /* If we're polling for input, don't get stuck in select for
 	     more than 25 msec. */
 	  struct timespec short_timeout = make_timespec (0, 25000000);