Cygwin: pipe: Avoid deadlock for non-cygwin writer.

Cygwin: pipe: Avoid deadlock for non-cygwin writer.

- As mentioned in commit message of the commit b531d6b0, if multiple
  writers including non-cygwin app exist, the non-cygwin app cannot
  detect pipe closure on the read side when the pipe is created by
  system account or the the pipe creator is running as service.
  This is because query_hdl which is held in write side also is a
  read end of the pipe, so the pipe is still alive for the non-cygwin
  app even after the reader is closed.

  To avoid this problem, this patch lets all processes in the same
  process group close query_hdl using newly introduced internal signal
  __SIGNONCYGCHLD when non-cygwin app is started.

  Addresses: https://cygwin.com/pipermail/cygwin/2022-March/251097.html