Flat network fields (bridge, ip, prefix, gateway, vlan) are now converted
into a single-entry interfaces[] list during normalization. All virtualization
tasks (proxmox, vmware, libvirt, xen) and configuration (NM, Alpine, Void)
now consume system_cfg.network.interfaces exclusively for multi-NIC support.
Also fixes: user.key -> user.keys in system_cfg output, strict list-only DNS
in example inventories, removes legacy single-MAC virtualization_mac_address
default.
