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