DEV-40930: Introduce LLAppViewerListener "forceQuit" operation.

The bug was driven by the following sequence in llstartup.cpp:
            if(reason_response == "update"
                || reason_response == "optional")
            {
                // used to resend status event still containing "update",
                // erroneously instantiating a second forced-update
                // LLAlertDialog
                LLLoginInstance::getInstance()->disconnect();
                // quit with an LLAlertDialog still on sModalStack used
                // to result in LL_ERRS
                LLAppViewer::instance()->forceQuit();
            }
I hope to be able to introduce a test script to verify the fix. That
script would need to be able to call LLAppViewer::forceQuit() rather than
requestQuit(), which is already available via LLAppViewerListener.

At the same time, changed LLAppViewerListener to bind a functor to
retrieve an LLAppViewer instance (namely LLAppViewer::instance) rather
than an LLAppViewer*. Apparently the static instantiation of
LLAppViewerListener was calling LLAppViewer::instance() too early, before
things were ready, so the declaration was changed to pass NULL -- then
in each method, call LLAppViewer::instance() if the bound pointer is NULL.
Binding the LLAppViewer* is a Feathers tactic intended to avoid the need
to reference the singleton. Binding a functor still leaves it up to the
instantiating code to reference LLAppViewer::instance, while deferring
the actual call to that method.