mpi/pmix: Continue code refactoring

- Introduce nitation of endpoint to distinguish between
  broadcast and point-to-point exchanges.
- Fix magic number naming: change all PMIX_ prefixes to PMIXP_
  to avoid conflicts with PMIx library macroses.
- Fix I/O engine cleanup logic as now I/O engines are used more
  than once.
- make a namespace subsystem to return xmalloc'd pointers as opposed
  to malloc'd to avoid confusion as everything else is xmallox'd.

Signed-off-by: Artem Polyakov <artpol84@gmail.com>