Add wait(CtrlCHandler&)

[bernardnormier]

This PR adds a new Ice::wait(CtrlcHandler&) function and updates the doc-comments for CtrlCHandler.

With this new wait function, a typical C++ server is as follows - we don't need communicator->waitForShutdown:

int
main(int argc, char* argv[])
{
    // CtrlCHandler handles Ctrl+C and similar signals. It must be created at the beginning of the program, before
    // starting any thread.
    Ice::CtrlCHandler ctrlCHandler;

    const Ice::CommunicatorHolder communicatorHolder{argc, argv};
    const Ice::CommunicatorPtr& communicator = communicatorHolder.communicator();

    auto adapter = communicator->createObjectAdapterWithEndpoints("GreeterAdapter", "tcp -p 4061");
    adapter->add(make_shared<GreeterServer::Chatbot>(), Ice::stringToIdentity("greeter"));
    adapter->activate();

    // Wait until the user presses Ctrl+C.
    wait(ctrlCHandler);
    cout << "done" << endl;

    return 0;
}

[externl]

Why not just have wait be a member function on CtrlCHandler, like std::condition_variable's wait().

I'm not sure I see the benefit of having a non-member function in this case as there's no abstraction of any kind.