libpriv/postprocess: target for oxidation

[lucab]

Postprocessing logic is a good target for next oxidation steps (i.e. 1:1 porting of the existing logic to Rust), because:

• it is intertwined with nssfiles/sysusers logic. Porting this first should make Drop requirement on nss-altfiles, use systemd sysusers #49 easier to tackle later on.
• it already calls into a lot of logic which is already in the Rust library. Porting this should allow dropping some cxxbridge compatibility methods.
• it would make Update auto-tmpfiles code to skip already specified directories #26 easier to pursue. Specifically, the fragment parsing part and the de-duplication logic could be less of a minefield.