bpo-36915: regrtest always remove tempdir of worker processes (GH-13312)

When using multiprocessing (-jN option), worker processes now create
their temporary directory inside the temporary directory of the
main process. So the main process is able to remove temporary
directories of worker processes even if they crash or when they are
killed by regrtest on KeyboardInterrupt (CTRL+c).

Rework also how multiprocessing arguments are parsed in main.py.