Shell: Make `quit` an alias for `exit`

Currently, if a user types `quit` into the shell, they are presented with this response:

> quit

function quit() {

    [native code]

}

Which is unhelpful, to say the least.  Of course, humans are clever and will try `exit` next and succeed.  Might even try calling quit() like a function and succeed that way too.  However, why not save that extra step and accept `quit` as a bare command helper?