util.h: clean up UTIL_INC_x and UTIL_DEC_x

Move the repetitive definitions used to add and subtract 1 at
preprocessor time into their own file. Make the behavior consistent,
so that you can invoke UTIL_INC(x) for any x you can invoke
UTIL_DEC(x) on.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>