ar.pm: Don't corrupt tables of symbols and long filenames
Don't reset to "644" the pseudo-permissions of the symbols table '/'. Whatever the placeholder is ("0" with GNU ar 2.31), messing with it diverges with the output of GNU ar which is already deterministic.
Long filenames are stored in a special '//' section with a completely different format. Stop assuming it has regular metadata and stop corrupting it.
Split the fixes in different commits so the tests can be more easily be tested.