[Gc] Interior pointers

[Florian Weimer]

* Ivan Maidanski:

>> In version 7, do interior pointers always keep objects alive if they
>> have been allocated with GC_malloc or GC_malloc_atomic?

> This should be the same as before.

Oh, I see that GC 6.x had GC_malloc_ignore_off_page, too.

I'm not sure what what's the difference between
GC_malloc_ignore_off_page and GC_malloc and how it interacts with
GC_set_all_interior_pointers).