support DPKG_ROOT

Johannes Schauer Marin Rodriguesrequested to merge
josch/openssh:dpkg_root into master

This is a draft merge request to maybe gain early feedback. The two problems from our side are how the group of /usr/bin/ssh-agent is set and that how ucf is called depends on what the ucf maintainer decides on debian/ucf!5 (closed)

The code was tested for correctness in the salsa CI pipeline associated to this MR: helmutg/dpkg-root-demo!8 (merged)

Having support for $DPKG_ROOT in openssh server is useful for easily setting up a buildd machine for new architectures where the foreign architecture is currently being bootstrapped and cannot be executed at all or only badly.

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