mount.nfs: rearchitect mount version/protocol negotiation logic

Text-based mounts try a mount operation first with default settings,
then negotiate via rpcbind queries and retry the mount, if the default
settings don't work.  This method introduces long delays in certain
common scenarios, and makes it difficult to tell when it is
appropriate to fail immediately or negotiate and retry.

To address these behavioral regressions, make text-based mounts
operate the same way that legacy mounts work.  Perform rpcbind queries
with short timeouts first, then use the results to determine
transport, version, and port number settings for the mount.

This allows the mount.nfs command to detect server settings, or
whether negotiation is even possible, quickly.  It also makes it
simple to determine when to fail vs. when to retry.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>

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