filer should expose only a basic shell

[modeswitch]

Filer's default shell should have a CWD, and expose the underlying FS API and that's all. Other shells should wrap the default shell to add functionality. Basic shell functionality like ls and cd is probably OK here, but things like zip should be omitted.

There's an open question here about whether shell scripts should live in the file system and be installed when the FS is formatted. There are advantages and disadvantages.