Static route preference incorrect for value 0

[Kenth Eriksson]

When setting preference value of zero on a static route, then this gets
shown as preference 200 when doing show route. Preference on static
routes is based on a patch from Ondrej with SHA
7a49e97e7ec86bd336dc801411cb8b8f92f38388, but it looks as this has
still not made into master.

kenth ~ # cat /opt/appl/executing/etc/bird_route.conf 
protocol static static1 { 
    ipv4 {import all;};
    check link 1;
    route 0.0.0.0/0 via 10.210.137.1 {preference=0;};
}

bird> show route 
Table master4:
0.0.0.0/0      unicast [static1 12:27:07.093] (200)
	via 10.210.137.1 on eth1

Setting preference to a value >= 1 makes this work correct.

/k