zebra: allow multiple connected routes for the same prefix

zebra: allow multiple connected routes for the same prefix

With unnumbered interfaces on Linux we have the same IP address configured
on several different interfaces and hence multiple connected routes for
the same prefix.

With that said, add an exception in rib_add() to allow zebra to keep
track of all connected routes. We don't need to worry about the bugs
reported in a3d18ce because connected routes are always added from the
connected_up() function, and connected_update() already takes care of
handling duplicate addresses per interface.

Fixes #1112.

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