Add __attribute__((const)) or __attribute__((pure)) to various
mach_read functions.
dtype_new_read_for_order_and_null_size(): Remove a statement
without effect: mach_read_from_2(buf + 4).
and modify some functions to return const pointers. Add const qualifiers
to local variable declarations or casts to remove the const qualifier
in those places where write access is needed.