NFS: set transport defaults after mount option parsing is finished

Move the UDP/TCP default timeo/retrans settings for text mounts to
nfs_init_timeout_values(), which was were they were always being
initialised (and sanity checked) for binary mounts.
Document the default timeout values using appropriate #defines.

Ensure that we initialise and sanity check the transport protocols that
may have been specified by the user.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>