vendor/mpir_pmi: Add some functions to use in MPIR_Session_reinit.

MPIR_mpi_staying_sync: Synchronize staying processes, block until all
leaving processes have unregistered with the PM and exited; polls
PMIX_QUERY_PROC_TABLE to check the status of all processes across all
known namespaces.

MPIR_pmi_get_number_nspaces: Provides number of known namespaces.

MPIR_pmi_get_nspaces: Provides a copy of the known namespaces names
as array of strings.