Cygwin: pty: Prevent deadlock on echo output.

- If the slave process writes a lot of text output, doecho() can
  cause deadlock. This is because output_mutex is held in slave::
  write() and if WriteFile() is blocked due to pipe full, doecho()
  tries to acquire output_mutex and gets into deadlock. With this
  patch, the deadlock is prevented on the sacrifice of atomicity
  of doecho().

winsup/cygwin/fhandler_tty.cc

@@ -496,11 +496,9 @@ void
 fhandler_pty_master::doecho (const void *str, DWORD len)
 {
   ssize_t towrite = len;
-  acquire_output_mutex (mutex_timeout);
   if (!process_opost_output (echo_w, str, towrite, true,
 			     get_ttyp (), is_nonblocking ()))
     termios_printf ("Write to echo pipe failed, %E");
-  release_output_mutex ();
 }
 
 int