Primary and secondary addresses depend on the order in which they
are added to an interface [1] .  This commit makes pyroute2 add IP
addresses in the same order Interface.add_ip is called.

This is achieved by changing LinkedSet.raw from a plain dict to an
OrderedDict and by reimplementing the set operators to preserve
ordering.

 [1] iproute2 docs: http://baturin.org/docs/iproute2/#Notes