Cygwin: pipe: Adopt FILE_SYNCHRONOUS_IO_NONALERT flag for read pipe.

Cygwin: pipe: Adopt FILE_SYNCHRONOUS_IO_NONALERT flag for read pipe.

- With this patch, FILE_SYNCHRONOUS_IO_NONALERT flag is applied to
  the read pipe so that native C# programs work with cygwin pipe.
  To handle signals in reading pipe with this flag set, the read
  pipe itself is always set to nonblocking mode and simulates the
  blocking behaviour in raw_read().

- Default to byte mode rather than message mode in order to make
  C# programs work with cygwin pipe.

Addresses: https://cygwin.com/pipermail/cygwin/2021-March/247987.html