Better handling for a multi-user setup

[yorickdowne]

What I did

Improve the use case where node:node-admins owns the directory, and alice and bob are members of node-admins

cp for created files becomes install, because I couldn't figure out how to be graceful otherwise when the user has group write permissions but forgot setgid

The base case of owner alice:alice and user alice has not been impacted. That's quite important, and if review finds that's not so, that needs to be fixed.

There are corner cases where bob, if they don't have sudo and other can't read on the directory, needs alice, who does have sudo, to run ethd at least once first, so config files are created and given the right permissions.

There is a test script at tests/test-multiuser.sh that runs through a number of scenarios and checks ownership and permissions. That includes running ethd as root and verifying that file ownership does not become root.

It's entirely plausible I missed a corner case. Careful review requested.

Also resolves #2570

[yorickdowne]

This'll conflict with #2569 and will need to be rebased after that's merged