Add support for Flex File Layout to the client.

This patch does not change the pNFS server, but adds Flex File Layout support
to the NFSv4.1 client. I have done this to the stable/11 branch so that others
can test more easily. This patch will go into head/current as a bunch of
smaller commits. As such, it will eventually be merged from head into
pnfs-planb-server.