tmpfiles: rework file attribute code

      - Stick to one type for the flags field: unsigned. This appears to be
  what the kernel uses, and there's no point in using something else.

- compress the flags array by avoiding sparse entries

- extend some error messages to not use abbreviated words

- avoid TTOCTTOU issues by invoking fstat() after open() when applying
  file flags

- add explanation why we need to check the file type with fstat().

- don't needlessly abbreviate "attribute" as "attrib", in particually as
  "chattr" abbreviates it as "attr" rather than "attrib".