Fix bug in sparse file listing

List posix archives containing sparse files >8GB correctly and do not fail.
This fixes also bug in format of listing for sparse files >8GB - now the
real size is printed instead of the effective one (this is not strictly
posix format related).

* src/list.c: Remove redundant assignment.
* src/tar.h: Add new 'real_size' and 'real_size_set' fields in
  tar_stat_info struct.
* src/xheader.c: Correctly handle (especially sparse) file sizes directly in
  xheader_decode().