Only use no_u_turn restriction for actual U-turns

Keepright hates when iD puts a `no_u_turn` restriction on a 12 degree angle:

![screenshot 2014-08-31 22 01 36](https://cloud.githubusercontent.com/assets/38784/4103957/fc0c1428-317b-11e4-8631-e9b89013d5b2.png)

I know it technically shouldn't matter (only the `no_` is significant as long as proper ways are assigned to the relation).   But it's currently easier to just change this in iD than in keepright.