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.