An NFSv4 only configuration does not register with
rpcbind(). Without this patch a failure to rpcb_unset()
is reported when the daemon is terminated for this case.
This is harmless noise, but this patch avoids calling
rpcb_unset() for the NFSv4 only case, avoiding the noise.
When called with "-d", it still does the rpcb_unset(),
assuming that the configuration might have been
changed to NFSv4 only and unregistering with
rpcbind() might still be needed.