[new-backend] resolver: add strategy for sudo user:/ namespace

[atmaxinger]

I have added a resolver strategy called s for the user:/ namespace. If the environment variable SUDO_USER is set, it will use passwd to resolve the path for this user. Unforunately, the tests do not work (yet), but this seems to be the case of the new-backend branch - not quite sure how to fix them yet.

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