Add working options for running Make instances in parallel

This removes the use of the non-working JOBS environment variable and
passes the arguments -j/--jobs to make directly. Additionally, the
Make option --load-average is now also supported.
Skip stand-alone npm installation by default

Node.js has included npm for a while now and it makes little sense to
install it separately by default. Hence, --without-npm has been
changed to --with-npm along with the related logic.

Associated parameter 'sanity checks' that don't make sense anymore
have been dropped as well.
Small changes to parameter name and description wording
Use strip() with --requirements instead of replacing just the newlines