Archives produced with GNU tar can have types of TypeGNULongName and
TypeGNULongLink.
These fields effectively appear like two file entries in the tar
archive. While golang's `archive/tar` transparently provide the file
name and headers and file payload, the access to the raw bytes is still
needed.
This fixes the access to the longlink header, it's payload (of the long
file path name), and the following file header and actual file payload.
