Since we switched from autotools to Meson in commit
94592c1406, 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.
These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.
No attributes are currently implemented.
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.
