Backout revision 1.56 and 1.57 of fifo_vnops.c.
Introduce a new poll op "POLLINIGNEOF" that can be used to ignore
EOF on a fifo, POLLIN/POLLRDNORM is converted to POLLINIGNEOF within
the FIFO implementation to effect the correct behavior.
This should allow one to view a fifo pretty much as a data source
rather than worry about connections coming and going.
Reviewed by: bde