This cleans the page handling when having to make an area copy-on-write;
regions belong to vm_cache_ref, pages to vm_cache.
Added prototype for new call vm_copy_area().
Fixed some return types.
git-svn-id: file:///srv/svn/repos/haiku/trunk/current@9263 a95241bf-73f2-0310-859d-f6bbb57e9c96