rc-file: Add "num-cores-low-mem" and move num-cores* defs to initial-setup.
It is annoying to be maintaining kdesrc-build-setup and --initial-setup but combining those two can be a subsequent refactoring. This adds a separate num-cores-low-mem to address the qtwebengine case and makes both num-cores and num-cores-low-mem into an option generated during initial setup, and only used by the default config (rather than any part of the kdesrc-build internals directly). There is a fallback reference in the code in case there is a usage of the default configuration file sections (e.g. qt5-build-include) but this is set conservatively (4 cores, 2 cores during low-mem). At this point it's almost "just" a configuration convention with a bit of code in the setup wizard so perhaps it's best not to touch the rest of the code/docs at all, but I'm happy with where this is at. I've tested --initial-setup in a Docker container (Fedora 29 with perl manually installed first) and tested kdesrc-build-setup separately.