When using the sync API entrypoints like fwupd_client_get_devices() these call
into fwupd_client_connect() to ensure the proxy is set up. This is not thread
safe and chaos ensues if you call two sync functions from different threads,
like GNOME Software does at startup...
Use a mutex to protect access to this shared resource.
Original test code from Philip Withnall, slightly tweaked by me. Thanks!
