Disallow updating of an existing installation

[bertm]

The installer cannot reliably be used to update an existing
installation, especially when dependencies have been updated: it does
not properly update wrapper.conf, breaking existing installations.

This exits the setup immediately when an existing Freenet installation
is found, displaying an error message to the user.

This requires the InnoSetup preprocessor (ISPP). I have tested this on Wine and it appears to work.

[ArneBab]

I think this error should only be shown, when the user tries to install into the same folder:

An existing Freenet installation is found in this folder. This installer cannot be used to update Freenet to a newer version. To update, enable Freenet's built-in automatic updates or run 'update.cmd' from the installation directory. To re-install, please uninstall Freenet first. To install another copy of Freenet, please choose another folder.

[bertm]

IMHO having multiple installations is an error for 99% of our users, and the other 1% will probably find their way around it.

Allowing multiple installations leads to more complex support scenarios, I'd prefer not having to deal with those.

[bertm]

That said, additional detection of an existing install based on a folder's contents (or just right out refusing to install into a non-empty directory) would not hurt.

[Thynix]

Superseded by #11.