nfscl: Handle the NFSERR_RETRYUNCACHEDREP error from a NFSv4 server

In a recent email list discussion related to NFSv4 mount problems
against a non-FreeBSD NFSv4 server, the reporter of the issue noted
that the server had replied 10068 (NFSERR_RETRYUNCACHEDREP). This
did not seem related to the mount problem, but I had never seen this
error before. It indicates that an RPC retry after a new TCP
connection has been established failed because the server did not
cache the reply. Since this should only happen for idempotent
operations, redoing the RPC should be safe.

This patch modifies the NFSv4.1/4.2 client to redo the RPC instead
of considering the server error fatal. It should only affect the
unusual case where TCP connections to NFSv4 servers are breaking
without the NFSv4 server rebooting.

Reported by: J David <j.devid.lists@gmail.com>
MFC after: 2 weeks