Set domain to TLD on GeoIP cookie

To spare both the Varnishes and the client the cost of additional lookups and
additional cookies, set the Domain= on the GeoIP cookie to the top-level
domain.

The domain always has a leading dot. Newer browsers ignore it, per RFC 6265,
but older browsers that adhere to the obsoleted RFC 2109 require it if the
cookie is to be valid for subdomains.

Change-Id: Ied7019e97a91d05d1228f692503ebea6d4afcc8a