net: if: Introduce carrier and dormant management on a network iface

Introduce a new interface state management scheme, according to
RFC 2863. This includes the following changes:

* Introduce a new interface flag: NET_IF_LOWER_UP, along with
  corresponding helper functions. The flag should be set/cleared on an
  interface by a network driver/L2 to signalize physical readiness of an
  interface to transmit data (for example cable plugged in).

* Introduce a new interface flag: NET_IF_DORMANT, along with
  corresponding helper functions. The flag should be set on an
  iterface when the interface is not ready to transmit application data,
  for example still not joined a Wi-Fi network.

* Introduce a new interface flag: NET_IF_RUNNING, indicating that
  interface is ready to transmit application data.

* Update the meaning of the NET_IF_UP flag - it now singnalizes whether
  an interface has been brought up/down by the application (admin
  up/down).

* Introduce operational state of an interface, derived from above. It
  reflects the internal interface state.

The meaning of net_if_is_up() function and NET_EVENT_IF_UP/DOWN events
remains unchanged to retain backward compability - they reflect the
interface readiness to transmit application data.

To verify the administrative up/down state, a new function
net_if_is_admin_up() has been introduced, along with
NET_EVENT_IF_ADMIN_UP/DOWN events.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>