add packaging date to .info files in package-release-nodes.php

in working on update_status 5.x-2.0 (which will soon be in core for D6), we decided we'd really like a datestamp in the .info files during packaging. then, we'd actually be able to warn sites that are using dev snapshots if a newer snapshot exists. we have a slight chicken in the egg problem, since we're putting this timestamp into the .info file *before* we tar it up, and it's the date stamp on the tarball that we actually store in the db as the file date field. however, these are only going to be at most a few seconds off from each other, and we only generate tarballs every 5 minutes (and only every 12 hours for dev snapshots). so, if we give ourselves a 100 second grace period or something, we're set.

the only open question is: what should we call the .info file directive? ;) originally, i was thinking "package_date", but then i realized that'd conflict with "package" in people's minds, which would be way too confusing. maybe just "datestamp" is best, since it's just a big integer. humans don't really need to try to understand it, it's just there for the machines, so it doesn't matter that much. if there are no serious objections, i'll commit this in the next day or so.