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.

winsup/cygwin/fhandler.h

@@ -1273,6 +1273,7 @@ class fhandler_fifo: public fhandler_base
   int nhandlers, nconnected;
   af_unix_spinlock_t _fifo_client_lock;
   bool reader, writer, duplexer;
+  size_t max_atomic_write;
   bool __reg2 wait (HANDLE);
   NTSTATUS npfs_handle (HANDLE &);
   HANDLE create_pipe_instance (bool);