gh-83714: Set os.statx().stx_mode to None if missing from stx_mask (#140484)

* Set stx_mode to None if STATX_TYPE|STATX_MODE is missing from
  stx_mask.
* Enhance os.statx() tests.
* statx_result structure: remove atime_sec, btime_sec, ctime_sec and
  mtime_sec members. Compute them on demand when stx_atime,
  stx_btime, stx_ctime and stx_mtime are read.
* Doc: fix statx members sorting.