Refactor: all: use func instead of FUNCTION

func is part of the C99 standard, while FUNCTION is an older GNU C
extension (that our portability.h also defined for the Sun C compiler).
However we used a mix of both already, meaning we already require both to
exist. So, just follow the standard.

This is the result of running the following, plus some manual clean-up:

make COCCI_FILES=coccinelle/use-func.cocci -C devel cocci-inplace