du: combining `-a`/`--all` and `-s`/`--summarize` should fail

[cakebaker]

Our implementation currently allows the combination of -a/--all and -s/--summarize whereas GNU du fails:

$ cargo run -q du -s -a
21380772        .
$ echo $?
0

$ du -s -a
du: cannot both summarize and show all entries
Try 'du --help' for more information.
$ echo $?
1