snapshot: cleanup and minor changes
- use OPAL_NAME to pack process name when OPAL_UINT64 was previously used
- move the definition of opal_process_name_t so an opal_value_t can have
  the OPAL_NAME type
- use opal_compare_proc() to compare process names when = != < > was previously   used with opal_identifier_t
- use opal_process_name_t in pmi instead of a private struct