This adds support for netdevs implementing the new API that provides
`netdev_driver_t::confirm_send()`. This allows implementing netdevs
in an event based non-blocking fashion, making live of driver
developers a bit easier. In addition, `gnrc_tx_sync` will now throttle
users of `sock_udp_send()` so that they can only send datagrams as
fast as the network stack and hardware is able to send out.
Finally, this lays the groundwork to fetch TX statistics (such as
TX timestamps, reception of layer 2 ACKs/NACKs, etc.) from the network
devices.
Adds a gnrc_netif specific rawmode flag to indicate that the netdev
device is configured in raw mode. This flag is kept in sync with a
possible flag in the netdev device and should only be modified via the
setter call.
With the previous fix, we only have to register addresses that are not
VALID yet on reception of router advertisements. This removes the need
for the hacky `GNRC_NETIF_FLAGS_6LO_ADDRS_REG` flag that was only
introduced to prevent unnecessary re-registration.
