grab.sh: Try and download a matched stage3/snapshot pair

Catalyst's autobuilds have been flaky recently, so it's a bit
difficult to predict when the next amd64 stage3 will land.  The
mirrors only keep a week of history, so it's easier to get the
stage3/snapshot pair from the same day if you try to grab them
automatically every evening.  This script will do that, using the
latest-stage3.txt to find the most recent stage3, and then downloading
the auxilliary files and Portage snapshot.  If the downloads all
succeed, it updates the default DATE in build.sh.