Add no_remove flag

[q3k]

This change adds a no_remove flag. Its purpose is to let users disable the atexit handler that removes the extracted archive when running a zip_safe=False parfile.

There's at least two scenarios when this is useful:

• post mortem analysis of malfunctioning a program/par
• environments when atexit might be spuriously called (eg. uwsgi workers being killed, while others are still running - I imagine this is due to multiprocessing)

Of course, the flag will cause disk space to get used, and the user needs to be aware of that.

[q3k]

@duggelz PTAL

[q3k]

cc @dkelmer @tmc