Detect old .multirust folders on install

[brson]

The installer already detects some problematic previous installations, but it does not notice when the old .multirust dir already exists. It should do so and refuse to install without intervention. Probably the best way to detect this is to look for the string "2" in .multirust/version.

This is made more subtle by the problem that the multirust uninstaller doesn't remove .multirust. So we have to tell them to delete it manually, or do it ourselves (which would be better).