avoid write-barrier memory protection for 'atomic-interior allocation

Split 'atomic-interior allocation to separate pages from other 'interior
allocation, and do not page-protect 'atomic-interior memory. Otherwise,
atomic-interior memory passed from one place to another --- especially
via the `#:in-original-place?` option on foreign functions --- can crash
due to triggering a write barrier in the wrong place.

Commit c18f6e8d6d, which changed some cross-place arguments to Pango and
Cairo to be 'atomic-interior, exposed the problem.

Merge to v6.0.1
ffi documentation: add overview section on places and thread-safety