printf: Explicit positive sign not parsed correctly (`printf "%d" +1`)

```
$ printf "%d %f\n" +1 +1
1 1.000000
$ cargo run printf "%d %f\n" +1 +1
printf: '+1': expected a numeric value
printf: '+1': expected a numeric value
0 0.000000
```

Interestingly, `seq` does parse this correctly. I guess that provides another reason to try to unify number parsing (#7458 is the first step of that).