avoid duplicate stats when searching for files in a path

avoid duplicate stats when searching for files in a path

* libguile/load.c (search_path): Extract from scm_search_path, to
  give the caller the stat buffer of the found path.
  (scm_search_path, scm_sys_search_load_path): Adapt accordingly.
  (compiled_is_fresh): Take the stat buffers directly.
  (scm_primitive_load_path, scm_init_eval_in_scheme): Adapt to
  search_path / compiled_is_fresh changes to avoid duplicate states in
  search-path.