Basic C++ logging interface

Basic C++ logging interface

Add C++ classes to replace md_logging.*, as well as (most if not all)
direct fplog and stderr output in the future.  This change contains the
implementation; child change(s) will actually take this into use once
the interface is final.

Documentation could still be improved, but at least an overview is there
(and the details may change somewhat once the usage spreads and there
are more use cases).

Change-Id: I8fc8550194853a5387057fdb7772f882fc061708