lang/ocaml: sort pkg-plist correctly, remove some old cruft

- Sort the *flat* pkg-plist (after the PLIST_SUB expansion), and drop
  needless ``@dir lib/ocaml/threads'' entry
- Remove "INSTALL* safeness" gimmick which had been added 16 years ago
  in r133975 and supposedly helped to preserve correct permissions in
  some circumstances, but these days it erroneously removes executable
  bits from some binaries, or causes nothing but stage Q/A warnings