oldnewthing-link-unfucker unfucks more effectively

I've fixed some issues in oldnewthing-link-unfucker, my tool for fixing old, broken links on The Old New Thing. Speficially, if you previously had it create a 404'ing link or lead to a post different from the one you were expecting, this should now fix the behaviour.

See the commit message for more in-depth discussion of the change.