Fix statfs to return individual total/used/free values to df(1).

Before this change, df(1) always reports total == used, even though the %free is calculated correctly.

If f_bfree is filled in, then df provides a correct triple of (total, used, free).

While here, go ahead and fill in f_frsize (since there's not really such a thing in gdrive, it can be == block_size).