RFC: make the experimental NFS subsystem the default one

[Rick Macklem]

> 
> The stablerestart(5) stuff needs to get dealt with before the
> switch-to
> experimental-server-as-default gets flipped on, but I know you'll get
> to
> that, Rick. If you need any testing/help from me, just let me know.
> 
Those changes are already committed to head, which is the only place where
the default will change. (I do plan on MFC'ing the stablerestart stuff.)

rick