topo/base: fix uninitialized variable

this commit fixes a bug introduced by commit open-mpi/ompi@e7c59e3adbcec5693c5df0057de6c66ed2b80817
Move process name {jobid,vpid} down to the OPAL layer.

* opal_process_name_t is now a struct :
    typedef uint32_t opal_jobid_t;
    typedef uint32_t opal_vpid_t;
    typedef struct {
        opal_jobid_t jobid;
        opal_vpid_t vpid;
    } opal_process_name_t;

* new opal_proc_table_t class :
  this is a hash table (key is jobid) of hash tables (key is vpid)
  and is used to store per opal_process_name_t info.

* new OPAL_NAME dss type

This commit is co-authored by Ralph and Gilles.