use BM_TARBALL_SLICESIZE with tar

[wntrstn]

It would be really useful to implement usage of BM_TARBALL_SLICESIZE (if set) when compressing with tar since dar doesn't support encryption (yet):

The encryption is not yet possible with "dar" archives.

---

As I get it from the tar man page, it should be quite painlessly implementable using the -L and -M switches:

-L, --tape-length NUMBER
      change tape after writing NUMBER x 1024 bytes

-M, --multi-volume
      create/list/extract multi-volume archive