This patch fixes systemd credential loading for wireguard
with networkd not working when specifying more than one
credential per interface by properly flattening the resulting
multidimensional list instead of falling back on string concat
with a space.
Resolves#365402
Enabling networking.wireguard.useNetworkd currently requires users to
modify the permissions of their private key files. Since that is a bad
upgrade experience, the module should be disabled by default for now.
Once systemd credential support is added to the module, it should be
safe to once again enable it by default for networkd users.
