Add robustness to failure of python multiprocessing

[petercorke]

Add robustness to failure of the Python standard multiprocessing module. This can happen for processes running in an suexec environment on a commercial cloud server where files in /sys are not readable, and leads to very unhelpful error messages and deep tracebacks.

The changes mean that if a thread pool can't be created then carry on in a single-threaded way.