Add O_NONBLOCK to the vn_open_cred() flags for NFS client locking when opening the POSIX fifo; convert ENXIO error returns to EOPNOTSUPP.
Add O_NONBLOCK to the vn_open_cred() flags for NFS client locking when
opening the POSIX fifo; convert ENXIO error returns to EOPNOTSUPP.

This improves handling of the case where the /var/run/lock fifo exists
but there is no listener: we immediately return EOPNOTSUPP rather
than blocking until a listener turns up.  This could occur during a
diskless boot before rpc.lockd is loaded, or if the lock file persists
across a reboot following the disabling of rpc.lockd.  This may have
suddenly started to occur due to fifo blocking fixes--previously it
looks like attempts to read on a fifo with no listener would time out
due to insufficient resources.

Reviewed by:	alfred

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase