Auto merge of #122812 - dtolnay:mode, r=workingjubilee

Show mode_t as octal in std::fs Debug impls

Example:

```rust
fn main() {
    println!("{:?}", std::fs::metadata("Cargo.toml").unwrap().permissions());
}
```

- Before: `Permissions(FilePermissions { mode: 33204 })`

- ~~After: `Permissions(FilePermissions { mode: 0o100664 })`~~

- After: `Permissions(FilePermissions { mode: 0o100664 (-rw-rw-r--) })`

~~I thought about using the format from `ls -l` (`-rw-rw-r--`, `drwxrwxr-x`) but I am not sure how transferable the meaning of the higher bits between different unix systems, and anyway starting the value with a leading negative-sign seems objectionable.~~

Author: bors