Path preview fixes

Fixes two tiny faults:
1) Pressing or releasing the strafe key does not currently update the currently previewed path.  Instead the unit moves ending up with a different number of TUs remaining than suggested by the preview.  Propose we update the preview instead.
2) With the strafe option set to false and using the strafe key regardless we get a path previewed with running costs used instead of walking ... 
![screen005](https://f.cloud.github.com/assets/1824834/1945796/656509e2-7fda-11e3-9c57-4c5cad05093a.png)
... with TU walking costs used
![screen006](https://f.cloud.github.com/assets/1824834/1945797/6a001ec4-7fda-11e3-8027-c25b7ced17e7.png)
This is fixed by checking the strafe option has been enabled.