Cleaning out rcfs/ckrm related operations in a backward compatible manner.

- on install do not write out /etc/fstab with /rfcs
- avoid trying to mount or umount /rcfs when it is not there

Doing this in a backwards compatibile manner so that we can still bring up
a PL system with a CKRM enabled kernel in the future (i.e., just for kicks),
although I suppose that will be unlikely.