Tightening the merge window rules

It's worth noting that the theoretical policy is not that Linus won't take new stuff after -rc1; it's that he won't take new stuff sent to him after -rc1. In order to make things easier to debug, he doesn't merge everything at once, and makes releases with only some of the changes, but (at least in theory) he's going through his backlog of things he got during the merge window for the major changes.