Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

With the new command, the decision to pause or close the output moves
into the output thread.

Prepare for adding another Cond attribute.

All it does is "sometimes" close the mixer, but that is also done in
the destructor.

This is implicitly done by the destructor.

Fixes shutdown memory leak.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

struct AudioOutput should not know or care about multi-threading.

Prepare to move some configuration settings from AudioOutput to
AudioOutputControl.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.