text-based mount.nfs: Implement falling back to NFSv2 and UDP

text-based mount.nfs: Implement falling back to NFSv2 and UDP

If the initial user-specified options fail (with EOPNOTSUPP or
EPROTONOSUPPORT) then the server has rejected the requested NFS version
or transport protocol.

In that case, probe the server, then construct a fresh set of mount
options that ask for the specific mountd and NFS version and transport
protocol that the server supports.  Rewrite the mount options based on
the results of the probe, then try the request again.

An additional kernel patch is required to cause the kernel to return
EOPNOTSUPP when an rpcbind fails during a NULL request.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>