gst-device-monitor: Detect shell and quote launch string

GstValue serialization is not the same as shell quoting, and raw
strings will also not yield a usable launch string due to shell
interpretation. We need to detect the shell and quote the launch
string accordingly.

Shell detection on Windows is done by looking at the parent process
while skipping python.exe and py.exe.

Fixes gstreamer/gst-plugins-base#922