vendor/mpir_pmi: Fallback to PMIx_Connect/Disconnect.

If the PMIx host does not support PMIx Process Groups, we
fall back to PMIx_Connect/Disconnect to synchronize the
active processes when malleability is initialized and
finalized.

Connect/Disconnect is only done if more than one namespace
is present, i.e. after a spawn operation. This avoids NOPs
as recommended by the PMIx standard.

In case of Connect/Disconnect we rely on the result of
PMIx_Query_info to create the global array of PMIx
processes that is used to determine the MPI rank to PMIx
proc ID mapping. This method assumes that the results
provided by PMIx_Query_info are exactly identical in all
processes.