refactor 55netcfg-copy-config
This adds tests for 55netcfg-copy-config using shUnit2, and then rewrites 55netcfg-copy-config to get rid of the repetitive stacks of if's.
This is has the networkd related changes removed, in order to allow it to cleanly merge onto the current master branch.
The networkd inclusive version is here: https://salsa.debian.org/philh/netcfg/-/tree/rebased_networkd?ref_type=heads