Simplify the definition of many build options.

Auto-stop the IO thread in all error handlers.

EXIT_SUCCESS and EXIT_FAILURE are defined in stdlib.h, not unistd.h.
D'oh!

The standard has been renamed since the early draft that was
implemented in MPD.

For type safety and code readability.

Move to fixed-point integers instead of floating point.

Reduce header dependencies.

For EXIT_SUCCESS.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Revert to the old API before commit e9e55b08, removing unnecessary
bloat.

Enables using range-based "for".

Move code from the "mad" plugin.

Move code from the "mad" plugin.