Cygwin: FIFO: code simplification

There are currently three functions that call NtQueryInformationFile
to determine the state of a pipe instance.  Do this only once, in a
new fifo_client_handler::set_state () function, and call that when
state information is needed.

Remove the fifo_client_handler methods pipe_state and get_state, which
are no longer needed.

Make fhandler_fifo::get_fc_handler return a reference, for use in
select.cc:peek_fifo.

Make other small changes to ensure that this commit doesn't change any
decisions based on the state of a fifo_client_handler.

The tricky part is interpreting FILE_PIPE_CLOSING_STATE, which we
translate to fc_closing.  Our current interpretation, which is not
changing as a result of this commit, is that the writer at the other
end of the pipe instance is viewed as still connected from the point
of view of raw_read and determining EOF.

But it is not viewed as still connected if we are deciding whether to
unblock a new reader that is trying to open.

winsup/cygwin/fhandler.h

@@ -1269,34 +1269,31 @@ public:
 
 #define CYGWIN_FIFO_PIPE_NAME_LEN     47
 
-/* The last three are the ones we try to read from. */
+/* We view the fc_closing state as borderline between open and closed
+   for a writer at the other end of a fifo_client_handler.
+
+   We still attempt to read from the writer when the handler is in
+   this state, and we don't declare a reader to be at EOF if there's a
+   handler in this state.  But the existence of a handler in this
+   state is not sufficient to unblock a reader trying to open. */
 enum fifo_client_connect_state
 {
   fc_unknown,
   fc_error,
   fc_disconnected,
   fc_listening,
-  fc_connected,
   fc_closing,
+  fc_connected,
   fc_input_avail,
 };
 
-enum
-{
-  FILE_PIPE_INPUT_AVAILABLE_STATE = 5
-};
-
 struct fifo_client_handler
 {
   HANDLE h;
   fifo_client_connect_state state;
   fifo_client_handler () : h (NULL), state (fc_unknown) {}
   void close () { NtClose (h); }
-/* Returns FILE_PIPE_DISCONNECTED_STATE, FILE_PIPE_LISTENING_STATE,
-   FILE_PIPE_CONNECTED_STATE, FILE_PIPE_CLOSING_STATE,
-   FILE_PIPE_INPUT_AVAILABLE_STATE, or -1 on error. */
-  fifo_client_connect_state &get_state () { return state; }
-  int pipe_state ();
+  fifo_client_connect_state set_state ();
 };
 
 class fhandler_fifo;
@@ -1462,7 +1459,7 @@ public:
   bool maybe_eof () const { return _maybe_eof; }
   void maybe_eof (bool val) { _maybe_eof = val; }
   int get_nhandlers () const { return nhandlers; }
-  fifo_client_handler get_fc_handler (int i) const { return fc_handler[i]; }
+  fifo_client_handler &get_fc_handler (int i) { return fc_handler[i]; }
   PUNICODE_STRING get_pipe_name ();
   DWORD fifo_reader_thread_func ();
   void fifo_client_lock () { _fifo_client_lock.lock (); }