support S_IMMUTABLE and S_APPEND file attributes inside a Container

Historically it was disabled because of the shared fs between Containers and inconvenience on Container destroy operation: if someone set a file immutable inside a CT, vzctl/prctl needs to remove the attr first and only after that remove data.

Now we store CT data inside a ploop => no problem on destroy => it seems to be safe to allow setting immutable inside a CT, but we still have simfs - shared fs case, so we need to distinguish these 2 cases, allow immutable for ploop and disable for shared fs (simfs) (new flag on superblock?).