Add a flag to control fix-modal-loop behavior

The change introduced by this branch can only work in a single-threaded
environment (blpwtk1, which was based on test_shell).  For blpwtk2, which is
based on the content module, this will not work and prevents work from being
done, because we won't have a constant timer to do work.

This commit makes this branch configurable.  In blpwtk1, we will call
DisablePostWorkInModalLoop, to enable to fix-modal-loop fix.  By default, it
will use the upstream behavior.