When an interface disappears, e.g. when a usb dongle gets unplugged,

When an interface disappears, e.g. when a usb dongle gets unplugged,
we get a RTM_IFANNOUNCE message not a RTM_IFINFO message.
Handle this message to not accumulate "unknown" interfaces.
While here fix a bug where we would remove an interface only in the
engine process but not in the frontend when an interfaces gets
unplugged while we process a RTM_IFINFO message for it.
OK benno