dpkg: use homebrew gtar and OSX md5, shasum

- The formula claims to depend on homebrew's gnu-tar package, but then
  uses OSX's bundled gnutar, rather than homebrew's gnutar.  On older
  OSX machines, this breaks, as dpkg 1.17 depends on tar >= 1.23, see
  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=642802

- dpkg-scanpackages fails, since md5sum, sha1sum, and sha256sum don't
  exist.  So replace the invocations with OSX's md5 and shasum tools.

Signed-Off-By: Drew Fisher <zarvox@zarvox.org>