Merge with 3.2: Issue #13158: Fix decoding and encoding of base-256 number fields...

Merge with 3.2: Issue #13158: Fix decoding and encoding of base-256 number fields in tarfile.

The nti() function that converts a number field from a tar header to a number
failed to decode GNU tar specific base-256 fields. I also added support for
decoding and encoding negative base-256 number fields.