Rework proc_parser_mft() further

Rework proc_parser_mft() further

By making proc_parser_mft_check() fail on a NULL manifest, we can
simplify the manifest selection logic further. This way we can see if
the new manifest has all the files it lists with correct hashes and
fall back to mft2 if not. This is still more complicated and uglier
than it should be, but far les convoluted and mind-bending than a day
ago.

ok job