Fix handling of options read from command line used as indirect option values

I'd like to get some feedback on a change to address #64 (closed) (trying to use something like --num-cores=2 at command line as a quick way to reduce CPU consumption by kdesrc-build).

The head of the work branch contains more info in the commit log which I don't want to retype here. But the change I thought was simple enough to test from a simple container turned out to be a bit more nuanced; everything in the new test case ended up being needed to catch one bug or another in versions of this patch as it evolved.