WaitingThread read some shared memory with the SubProcess
(`mutex`, `m_running`).
When we destroy the SubProcess, we must be sure that WaitingThread has
correctly finished else we may have invalid read/write on freed memory.
The `exit` function will call the functions registered.
Qt may (and does) register function and may (and does) hangup waiting
for some mutex to be free.
Here we are in a forked process and we want to process to exit without
doing any cleanup (because we are a clone of a process that will continue
and we don't want to mess it).
Matthieu Gautier