Improve pathname handling and canonicalized /usr move

Guillem Joverrequested to merge
guillem/resolvconf:pu/canonical-usr into unstable

This improves the general handling for third-party commands, and performs a full canonical move to /usr, which will be eventually needed anyway.

But if there are intentions to backport this version then the full move will not be doable right now I guess.

(I added a .gitignore file, because that was annoying during builds, but I can take it out and submit independently if you'd prefer.)

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