Correct constness of system attributes in heap.c & prerequisites.

This allows the compiler / linker to mark affected pages as read-only.

There's a fair number of pre-requisite changes, to allow the const
properly be propagated. Most of consts were already required for
correctness anyway, just not represented on the type-level.  Arguably
we could be more aggressive in using consts in related code, but..

This requires using a few of the types underlying typedefs that
removes pointers (e.g. const NameData *) as declaring the typedefed
type constant doesn't have the same meaning (it makes the variable
const, not what it points to).

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

src/backend/utils/adt/name.c

@@ -188,7 +188,7 @@ namege(PG_FUNCTION_ARGS)
 /* (see char.c for comparison/operation routines) */
 
 int
-namecpy(Name n1, Name n2)
+namecpy(Name n1, const NameData *n2)
 {
 	if (!n1 || !n2)
 		return -1;