btl sm: restore btl_sm_have_knem_support MCA param

Somehow, this MCA param was accidentally dropped after v1.6.5.  Thanks
to Gus Correa for bringing this matter to our attention.

Also moving some MCA params down from level 9 to levels 4/5.
btl sm: error if knem support is requested and cannot be activated

Restore the functionality to error out (and show a helpful message) if
knem support is requested by is either not compiled in or cannot be
activated.

Thanks to Gus Correa for bringing the matter to our attention.