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.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.

Another big chunk of code for multi-player support.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The special-purpose function is used for saving/restore the software
volume control to the state file.

Converted the range checks in volume_level_change() to assertions.
Changed all volume types to "unsigned", expect for those which must be
able to indicate error (-1).