shell: expose instance_owner in shell info object

Problem: The shell fetches the local instance owner, but does not
expose this value to plugins.

Add an instance_owner key to the shell info object.

Author: grondo

src/shell/shell.c

@@ -448,9 +448,10 @@ static json_t *flux_shell_get_info_object (flux_shell_t *shell)
         return o;
 
     if (!(o = json_pack_ex (&err, 0,
-                            "{ s:I s:i s:i s:i s:s s:O s:O s:{ s:i }}",
+                            "{ s:I s:i s:i s:i s:i s:s s:O s:O s:{ s:i }}",
                             "jobid", shell->info->jobid,
                             "rank",  shell->info->shell_rank,
+                            "instance_owner", (int) shell->broker_owner,
                             "size",  shell->info->shell_size,
                             "ntasks", shell->info->total_ntasks,
                             "service", shell_svc_name (shell->svc),

src/shell/shell.h

@@ -105,6 +105,7 @@ const struct taskmap *flux_shell_get_taskmap (flux_shell_t *shell);
 /*  Return shell info as a JSON string.
  *  {
  *   "jobid":I,
+ *   "instance_owner":i,
  *   "rank":i,
  *   "size":i,
  *   "ntasks";i,