Make debian control tarball use owner and group arguments (gnutar fix)

This PR is related to #433.

Problem:

If the group doesn't exist, `gnutar` (1.17) will fail on MacOSX (10.8.3).

Fix:

If on darwin, and `--group root` is specified, switch to using a numeric GID.

How to replicate:

      $> gnutar --owner root --group root
      gnutar: root: %s: Invalid group

      $> gnutar --owner root --group wheel
      gnutar: You must specify one of the `-Acdtrux' options

      $> gnutar --owner root --group 0 --numeric-owner
      gnutar: You must specify one of the `-Acdtrux' options

I merged [mmb commit](https://github.com/mmb/fpm/commit/b7479a107052ff0ea4fd9106febb64dafe24261a).

Let me know if more details are needed,

Thanks!