Move extend_info_from_router() from circuitbuild.c to router.c and
make it static.

Add get_configured_bridge_by_orports_digest() and have
get_configured_bridge_by_routerinfo() and
node_is_a_configured_bridge() use it. We now consider all OR ports of
a bridge when looking for it.

Move node_get_*_orport to nodelist.c.

Fix a cut'n'paste error in header of nodelist.h.

Add node_assert_ok().

Add router_get_all_orports(). It's duplicating code from
node_get_all_orports(). Worth fixing at the cost of complicating the
API slightly?