The STACK macros take care of casting to and from the designated item type

of the stack, and the (void *) type used in the underlying sk_***
functions.  However, declaring a STACK_OF(type) where type is a *function*
type implicitly involves casts between function pointers and data pointers.
That's a no-no. This changes the ENGINE_CLEANUP handling to use a regular
data type in the stack.