Cygwin: serial: revamp overlapped IO in read and select

Get rid of WaitCommEvent and using overlapped_armed to share the
same overlapped operation between read and select.  Rather, make
sure to cancel the overlapped IO before leaving any of these functions.

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

winsup/cygwin/fhandler.h

@@ -1687,7 +1687,6 @@ class fhandler_serial: public fhandler_base
   int dtr;				/* for Windows 9x purposes only */
 
  public:
-  int overlapped_armed;
   OVERLAPPED io_status;
 
   /* Constructor */