The interface destructor called pure virtual methods at least when
cancelling ARP- waiting states. The implementations were made by the
deriving classes Session_component respectively Uplink. This led to an
abort of the NIC router as the destruction of the derived class was
already done. A similar problem already occured in the past during the
construction of Interface and was back then solved by introducing a
separate init() method. This commit, however, solved the problem by
making Interface a member of the other classes. Therefore, the init()
method could be removed again. Furthermore, the interface polica could be
moved from Session_component_base to Session_component. The commit also
had to generalize the way the link state of an interface is determined.
Fixes#2856
97ea513122