macOS: Intermittent hang in qt_safe_accept

On macOS, the ::accept call in qt_safe_accept intermittently blocks indefinitely in the attached code, causing a hang.

To demonstrate in the attached code, build and run in the terminal (and Ctrl+C to quit):

./NCA-9481-widgets.app/Contents/MacOS/NCA-9481-widgets

After several repeats, the hang will occur. It seems that qt_safe_accept sometimes gets called spuriously a second time, and ::accept blocks. Adding O_NONBLOCK to the qt_safe_socket call in QLocalServerPrivate::listen(const QString &) makes the hang go away.