Cygwin: FIFO: update select

Cygwin: FIFO: update select

Add static functions peek_fifo, thread_fifo, start_thread_fifo, and
fifo_cleanup to select.cc.  These are based on the corresponding pipe
functions, the main difference being that peek_fifo loops through the
connected clients to see if any of them have data available for
reading.

Add the fhandler_fifo methods select_read, select_write, and
select_except.

Add accessor methods get_nclients, get_handle, and is_connected that
are needed by peek_fifo.