[mfdinupg] Directory quotas and cleanup hooks

It would be nice if user-level filesystem tools could set a quota on a directory.

Perhaps this could be "faked" with a user IO API layer, with all processes accessing the directory voluntarily going through the API.  It could be tricky with distributed filesystems.

Vaguely inspired as a shortcoming of AFS.  Although volumes have quotas, creating and manipulating volumes is not accessible to the user.  We would also like not to commit empty space to a volume.

Even nicer would be a hook such that, if usage hits the max quota, the system automatically invokes a hook to clean up space; perhaps delete the oldest data.