Based on questions and comments from dgoulet, I've tried to fill
in the reasoning about why these functions work in the way that they
do, so that it will be easier for future programmers to understand
why this code exists and works the way it does.
Since we can't be sure that we can unlink enough files on windows
here, let's let the number of permitted entries grow huge if it
really must.
We do this by letting the storagedir hold lots of entries, but still
trying to keep the number of entries under the configured limit. We
also have to tell consdiffmgr not to freak out if it can't actually
remove enough entries.
Part of a fix for bug 22752
