Create a tarball of planet-heads and use it in the review script

* Create the planet-heads.tar file to be stored on the server side
* Document the tarball in .htaccess.var and README files
* Download the tarball within the review-planet-config script and move
  the heads from it to the previously cleared planet-git/heads/deriv
  directory
* Add curl --config option and make other options look GNU-like

Earlier new versions of the existing heads were not downloaded correctly
because of the wget -nc option. Head deletions weren't shown either.
Now the planet-git/heads/deriv directory contains the correct set of heads
and new heads are transferred from deriv.debian.net with one download.

Fixes: commit b10f9413