mdbx: refine freelist_save() to simplify loose-pages handling (preparation for pages refunding).
mdbx: refine freelist_save() to simplify loose-pages handling (preparation for pages refunding).

Change-Id: I1f15487383fbfb6770e1f7128a2d8349a1cfdf25

leo-yuriev/libfpta,leo-yuriev/libfpta