SFTP: should removing a non-empty directory be permitted?

[davidsarah]

The SFTP frontend currently allows deleting a non-empty directory. This is a well-defined and useful operation on a Tahoe filesystem, but it would not be allowed on a POSIX filesystem.

It's unclear whether applications using SFTP via sshfs might rely on attempts to delete a non-empty directory being denied. In principle they could, but I've no evidence that this is common.

TODO: check whether rm [-r] [-f] on platforms that support sshfs explicitly checks that a directory is empty before trying to delete it.