Auto merge of #5196 - matklad:clapclapclap, r=alexcrichton

Fix a regression with parsing multivalue options

By default, clap interprets

```
cargo run --bin foo bar baz
```

as

```
cargo run --bin foo --bin bar --bin baz
```

This behavior is different from docopt and does not play nicely with
positional arguments at all. Luckily, clap has a flag to get the
behavior we want, it just not the default! It will become the default in
the next version of clap, but, until that time, we should be careful
when using the combination of `.long`, `.value_name` and
`.multiple(true)`, and don't forget to specify `.number_of_values(1)` as
well.

@alexcrichton I'd love to merge this fix before updating cargo at rust-lang/rust :)