Cygwin: ioctl: TIOCINQ: always return number of chars in the inbound queue

So far ioctl(TIOCINQ) could end up returning -1 with errno set to EINVAL
if a non-zero device error mask has been returned by ClearCommError.
This doesn't reflect Linux behaviour, which always returns the number of
chars in the inbound queue, independent of any I/O error condition.
EINVAL was a pretty weird error code to use in this scenario, too.

Fix this by dropping all checking for device errors in the TIOCINQ
case.  Just return the number of chars in the inbound queue.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_serial.cc

@@ -519,13 +519,6 @@ fhandler_serial::ioctl (unsigned int cmd, void *buf)
 	  }
 	break;
      case TIOCINQ:
-       if (ev & CE_FRAME || ev & CE_IOE || ev & CE_OVERRUN || ev & CE_RXOVER
-	   || ev & CE_RXPARITY)
-	 {
-	   set_errno (EINVAL);	/* FIXME: Use correct errno */
-	   res = -1;
-	 }
-       else
        ipbuf = st.cbInQue;
        break;
      case TIOCGWINSZ:

winsup/cygwin/release/3.1.5

@@ -9,3 +9,6 @@ Bug Fixes:
 - Fix a segfault when starting, e.g., mintty from a bash in a console
   running xterm emulation.
   Addresses: https://cygwin.com/ml/cygwin/2020-02/msg00197.html
+
+- Fix TIOCINQ to always return number of inbound chars if available.
+  Addresses: https://cygwin.com/ml/cygwin/2020-02/msg00258.html