ssh refuses to run under oniux (user_ns don't map root)

reported on irc, ssh doesn't work with oniux.

After testing myself, cargo run -- ssh abc errors out with /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf (some pre-installed rules to connect to systemd-machine easily). The file is readable, but its uid/gid are 65534/65534, instead of the expected 0/0, likely because of userns. ssh doesn't like its files being writable by anyone but root and the current user, so seeing anything else here scares it out.

note: simply mapping 0 to 0 is probably a solution, but that might not be a good idea depending on the rational for using userns