kvs: fix logic issue in lookupat

[chu11]

Was talking to @garlick and he was wondering why in lookup_create() there is a root_dir and a root_ref argument.

lookup_t *lookup_create (struct cache *cache,
                         int current_epoch,
                         const char *root_dir,
                         const char *root_ref,
                         const char *path,
                         flux_t *h,
                         int flags);

I remembered it was for a specific corner case where the user passes in a at-reference via lookupat and the user specifies as key of .. The current code returns the "real root reference" as a response. It should instead return the at-reference.

As a consequence of this fix, we can eliminate one of the above arguments in lookup.