Interface logic changes:

o  Introduced IF_LINK_UP flag corresponding to real link state.
o  Allowed addressless interfaces.
o  IF_UP is now automatically calculated and set iff the interface
   is administratively up, has link up and has an IP address assigned.
   It may be IF_IGNORED, though (as in case of the loopback).
o  Any changes which include up/down transition are considered small
   enough to not provoke artificial upping and downing of the interface.
o  When an interface disappears (i.e., it wasn't seen in the last scan),
   we announce this change only once.
o  IF_LOOPBACK implies IF_IGNORE.