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

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.