Allow uninstalling user shortcuts as administrators

[marcoesters]

Description

menuinst can install shortcuts into user-specific or system locations, depending on whether the user is administrator or not and whether the .nonadmin file is in the target or the base prefix. If the user is administrator, .nonadmin files are ignored. Uninstalling shortcuts follows the same principle: menuinst searches for system locations if the user is administrator or if no .nonadmin files are present.

This causes an inconsistency if the user is not an administrator at installation, but an administrator at uninstallation. This is not an uncommon scenario since uninstallers may be invoked with elevated privileges by default. This causes shortcuts to be left behind.

This PR implements a heuristic for uninstallations to check user locations if:

• The user is administrator
• No system shortcuts have been removed
• A .nonadmin file is present in the target or the base prefix.

Closes #341

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Nice! You are changing the returned values so they are only returned if they actually existed and were removed. This allows to check whether something happened as an admin, and fallback to user shortcuts if the installation paths are marked as .nonadmin.

I only have a few suggestions about actually checking for files and not just existence, because otherwise the unlink calls may fail when passed a directory by accident.