Merge branch 'lsns--refactor-related-to-2987' of https://github.com/masatake/util...

* 'lsns--refactor-related-to-2987' of https://github.com/masatake/util-linux:
  lsns: (refactor) use ls_path_{openf|statf} to make the code simple
  lib/path: add ul_path_statf() and ul_path_vstatf()
  lsns: verify the uniqueness of a namespace in ls->namespaces list
  lsns: (refactor) make the function names for reading namespaces consistent
  lsns: (refactor) rename read_related_namespaces to connect_namespaces
  lsns: (refactor) rename get_ns_ino() to get_ns_inos()
  lsns: (refactor) use get_{parent|owner}_ns_ino() in add_namespace_for_nsfd
  lsns: (refactor) add get_{parent|owner}_ns_ino() implementing some parts of get_ns_ino()
  lsns: (refactor) give a enumeration name 'lsns_type' to LSNS_TYPE_ enumerators
  lsns: (refactor) rename LSNS_ID_.* to LSNS_TYPE_.*