iface-helper: add nm-iface-helper for dynamic configure-then-quit support
When quitting, the Manager asks each device to spawn the interface helper, which persists and manages dynamic address on the interface after NetworkManager is gone. If the dynamic address cannot be maintaned, the helper quits and the interface's address may be removed when their lifetime runs out. To keep the helper as simple as possible, NetworkManager passes most of the configuration on the command-line, including some properties of the device's current state, which are necessary for the helper to maintain DHCP leases or IPv6 SLAAC addresses.
Showing with 865 additions and 72 deletions