btl/vader: unlink the shared memory segment when finished

cmr=v1.8:reviewer=jsquyres
osc/base: adjust the logic in ompi_osc_base_sndrcv_op to adjust for
the case fix in ompi_osc_base_process_op in r31204.

There are two cases that needed to be handled:

 - The target is a simple datatype (contiguous block of a primitive
   type) but the origin is not. In this case we still need to pack
   the origin data but we can not rely on the convertor to do the
   unpack (see r31204).

 - Both the origin and target datatypes are simple datatypes. In this
   case we can use ompi_op_reduce to do the accumulation without having
   to pack the origin data.

cmr=v1.8:ticket=#4449