Cygwin: FIFO: improve raw_write

Cygwin: FIFO: improve raw_write

Don't set the write end of the pipe to non-blocking mode if the FIFO
is opened in blocking mode.

In fhandler_fifo::raw_write in blocking mode, wait for the write to
complete rather than returning -1 with EAGAIN.

If the amount to write is large, write in smaller chunks (of size
determined by a new data member max_atomic_write), as in
fhandler_base_overlapped.

For convenience, add two new NTSTATUS codes, STATUS_THREAD_SIGNALED
and STATUS_THREAD_CANCELED, to ntdll.h.