Cleanup: polish Config2 using C++ features

Cleanup: polish Config2 using C++ features

We now seem to have had several patches successfully use members
declared with default values (C++11 feature) and/or with the
"*this = Foo();" shortcut for a reset/clear method.

So I think we can start using these to replace old C-style
initialization and clear() functions.

This patch begins by replacing the Config2 use of memset(). I for one am
constantly mistaking which of the Config objects has memset() applied to
it at the global level when reconfigure happens. Now we do not need to
care, each object handles its own clearing one way or another.