Cygwin: FIFO: fix problems finding new owner

Cygwin: FIFO: fix problems finding new owner

When the owning reader closes and there are still readers open, the
owner needs to wait for a new owner to be found before closing its
fifo_client handlers.  This involves a loop in which dec_nreaders is
called at the beginning and inc_nreaders is called at the end.  Any
other reader that tries to access shmem->_nreaders during this loop
will therefore get an inaccurate answer.

Fix this by adding an nreaders method and using it instead of
dec_nreaders and inc_nreaders.  Also add nreaders_lock to control
access to the shmem->_nreaders.

Make various other changes to improve the reliability of finding a new
owner.