RFC: prevent MFS write to /ipfs at root

[alanshaw]

cp and ls have the issue of not being able to distinguish between an IPFS path and an MFS path.

If we prevent any writes (including cp, mv etc.) to /ipfs there will never be any data at /ipfs to cp or ls from and we avoid the issue of having to distinguish i.e. if it begins with /ipfs then it's an IPFS path, since no writes can go to /ipfs in MFS.

[olizilla]

I read "restrict to" as "only allow to". I think this would make more sense as Prevent MFS write to /ipfs