Prefix all SPL debug macros with 'S'

Prefix all SPL debug macros with 'S'

To avoid conflicts with symbols defined by dependent packages
all debugging symbols have been prefixed with a 'S' for SPL.
Any dependent package needing to integrate with the SPL debug
should include the spl-debug.h header and use the 'S' prefixed
macros.  They must also build with DEBUG defined.