fcm make: steps: don't inherit if already set

Hence, allow `steps=` to be declared before `use=`.

In addition:
* incr mode: on load of prev ctx, remove prev ctx of irrelevant steps.
* config unparse: `use=` before `steps=`.
Merge pull request #101 from matthewrmshin/fix-fcm-make-inherit-steps

fcm make: steps: don't inherit if already set