showmount: Try the highest mount version then fall back to lower ones

Showmount should try the highest mount version first then fall
back to the lower ones when the server returns a RPC_PROGVERSMISMATCH
error. The idea being not using the lower mount versions will begin
the process of moving away from NFSv2 support.

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