Avoid deadlock when calling subprocess.Popen()

[m3nu]

This (hopefully) avoids the deadlock situation mentioned in the Python docs and this SO question:

The preexec_fn parameter is not safe to use in the presence of threads in your application. The child process could deadlock before exec is called. If you must use it, keep it trivial! Minimize the number of libraries you call into.

In addition it avoids getting the details of terminated Borg processes when looking for mount points. Discussed here.