remove improper use of hwloc_bitmap_free

When using the native aprun launcher, it was observed that
there were frequent memory corruption errors occuring either
during a PMI kvs-fence operation, or at mpi termation during
opal cleanup of allocated objects.  This was especially bad
when using

aprun --c none

In some cases, the application would even just hang in finalize
if using ptmalloc, owing to some kind of infinite loop in
cleanup of small blocks, etc.

It turns out that the proble was in orte_ess_base_proc_binding's
improper use of opal_hwloc_base_get_available_cpus.  The cpuset
(bitmap) returned from that function is not meant to be freed
by the caller.

This problem is likely never observed when using the mpirun launcher
as there's an early exit if the OMPI_MCA_orte_bound_at_launch
environment variable is set.
fix setting of PMIX_NODE_RANK in cray pmix comp.

Per discussions with pmix folks, it was determined that
the way the cray pmi pmix component was computing the
PMIX_NODE_RANK attribute for a process was incorrect.
This commit fixes the problem.