addresses per interface (needed for example for IPv6 support).

Visible changes:

o  struct iface now contains a list of all interface addresses (represented
   by struct ifa), iface->addr points to the primary address (if any).
o  Interface has IF_UP set iff it's up and it has a primary address.
o  IF_UP is now independent on IF_IGNORED (i.e., you need to test IF_IGNORED
   in the protocols; I've added this, but please check).
o  The if_notify_change hook has been simplified (only one interface pointer
   etc.).
o  Introduced a ifa_notify_change hook. (For now, only the Direct protocol
   does use it -- it's wise to just listen to device routes in all other
   protocols.)
o  Removed IF_CHANGE_FLAGS notifier flag (it was meaningless anyway).
o  Updated all the code except netlink (I'll look at it tomorrow) to match
   the new semantics (please look at your code to ensure I did it right).

Things to fix:

o  Netlink.
o  Make krt-iface interpret "eth0:1"-type aliases as secondary addresses.