kvs: use optional key when unpacking json

Problem: In lookup_common() multiple calls to flux_request_unpack() are
made for optional json keys.  This is likely legacy from before jansson's
"?" optional flag was available.

Remove the excess calls to flux_request_unpack(), using the "?" optional
flag as needed.

Author: chu11

src/modules/kvs/kvs.c

@@ -1317,25 +1317,17 @@ static lookup_t *lookup_common (flux_t *h, flux_msg_handler_t *mh,
         struct flux_msg_cred cred;
         int root_seq = -1;
 
-        if (flux_request_unpack (msg, NULL, "{ s:s s:i }",
+        /* namespace, rootdir, and rootseq optional */
+        if (flux_request_unpack (msg, NULL, "{ s:s s:i s?s s?o s?i}",
                                  "key", &key,
-                                 "flags", &flags) < 0) {
+                                 "flags", &flags,
+                                 "namespace", &ns,
+                                 "rootdir", &root_dirent,
+                                 "rootseq", &root_seq) < 0) {
             flux_log_error (h, "%s: flux_request_unpack", __FUNCTION__);
             goto done;
         }
 
-        /* namespace is optional */
-        (void)flux_request_unpack (msg, NULL, "{ s:s }",
-                                   "namespace", &ns);
-
-        /* rootdir is optional */
-        (void)flux_request_unpack (msg, NULL, "{ s:o }",
-                                   "rootdir", &root_dirent);
-
-        /* rootseq is optional */
-        (void)flux_request_unpack (msg, NULL, "{ s:i }",
-                                   "rootseq", &root_seq);
-
         /* either namespace or rootdir must be specified */
         if (!ns && !root_dirent) {
             errno = EPROTO;