Cygwin: console: Introduce new thread which handles input signal.

Cygwin: console: Introduce new thread which handles input signal.

- Currently, Ctrl-Z, Ctrl-\ and SIGWINCH does not work in console
  if the process does not call read() or select(). This is because
  these are processed in process_input_message() which is called
  from read() or select(). This is a long standing issue of console.
  Addresses:
    https://cygwin.com/pipermail/cygwin/2020-May/244898.html
    https://cygwin.com/pipermail/cygwin/2021-February/247779.html

  With this patch, new thread which handles only input signals is
  introduced so that Crtl-Z, etc. work without calling read() or
  select(). Ctrl-S and Ctrl-Q are also handled in this thread.