Validate users variables that are not the first path segment

[joachimmetz]

Per #306

In GRR we use a regex to get the interpolations our, have a look at
https://github.com/google/grr/blob/8123918157e0c53a2ba36d225cec542c6ee4524d/grr/core/grr_response_core/lib/artifact_utils.py#L75
Maybe you can reuse that code?

[joachimmetz]

%%users.appdata%%
%%users.homedir%%
%%users.localappdata%%
%%users.sid%%
%%users.temp%%
%%users.username%%
%%users.userprofile%%