"Error while elevating rights" on different OS

Installation to locations requiring additional privileges reproducably fails with "Error while elevating rights" on at least Mac, Linux.

The issue is a race condition: Since change https://codereview.qt-project.org/#change,38167 the UI process doesn't wait any more infinitely for the fs process to come up, but tries to connect to the given port 3 times ... if that fails it gives up and shows above error message. The 'socket->waitForConnected(10000)' in FSEngineClientHandler::connect seems to indicate that it tries to connect for at least 30 seconds, but actually waitForConnected() returns immediately for ConnectionRefused - the error when the port isn't (yet) open.