Unix domain/FIFOs: Fix a race condition between FIFO buffer operations and...

Unix domain/FIFOs:  Fix a race condition between FIFO buffer operations and the opening and closing of FIFOs which necessary when the FIFOs are used to support Unix domain, datagram sockets.  The default policy is the deallocate FIFO buffering when the last client closes the pipe.  When when used for datagram communicatinos, packets left in the FIFO will be lost.  Some like UDP read-ahead is needed:  The buffered data in the FIFO needs to be retained until the reader gets a chance to re-open the FIFO.  Added an ioctl (PIPEIOC_POLICY) to control the buffer policy. Default (0) is the legacy behavior; Unix domain datagram logic sets the alternative policy so that the packet data persists after the FIFO is closed.