Make sure disabling old RSA keys works on Windows.

Make sure disabling old RSA keys works on Windows.

Seeking in files and rewriting parts of them does not seem to work properly on
Windows. Instead, when old RSA keys are found when generating new ones, the
file containing the old keys is copied to a temporary file where the changes
are made, and that file is renamed back to the original filename. On Windows,
we cannot atomically replace files with a rename(), so we need to move the
original file out of the way first. If anything fails, the new code will warn
that the user has to solve the problem by hand.