modules/kvs: rework kvsdir_get_*() functions

If a kvsdir_t was created with kvs_get_dirat(), then the
convenience functions kvsdir_get_*() operate "at" (relative to)
the original treeobj.

This means a recursive walk using kvsdir_get_dir() will
follow a consistent snapshot, unaffected by changes being
made concurrently to the KVS namespace.

Fixes flux-framework#64