*: use rb-trees to store interfaces instead of sorted linked-lists

*: use rb-trees to store interfaces instead of sorted linked-lists

This is an important optimization for users running FRR on systems with
a large number of interfaces (e.g. thousands of tunnels). Red-black
trees scale much better than sorted linked-lists and also store the
elements in an ordered way (contrary to hash tables).

This is a big patch but the interesting bits are all in lib/if.[ch].

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>