Properly handle interrupted build processes.

When a build has been interrupted by the user (Ctrl-C),
a debugging shell has been entered.
That's not what a user would expect, so we just end the
build process.
Remove unused variable "error".
Translation updates.

New language: Dutch.
Bump version to 0.9.24.
A new try to get the parallelism straight.

Getting back to CPU cores * 2.