Manual Debug for Unix ExitCode ExitStatus ExitStatusError

These structs have misleading names.  An ExitStatus[Error] is actually a Unix wait status; an ExitCode is actually an exit status.  These misleading names appear in the `Debug` output.

The `Display` impls on Unix have been improved, but the `Debug` impls are still misleading, as reported in rust-lang#74832.

Fix this by pretending that these internal structs are called `unix_exit_status` and `unix_wait_status` as applicable.  (We can't actually rename the structs because of the way that the cross-platform machinery works: the names are cross-platform.)

After this change, this program
```
#![feature(exit_status_error)]
fn main(){
    let x = std::process::Command::new("false").status().unwrap();
    dbg!(x.exit_ok());
    eprintln!("x={:?}",x);
}
```
produces this output
```
[src/main.rs:4] x.exit_ok() = Err(
    ExitStatusError(
        unix_wait_status(
            256,
        ),
    ),
)
x=ExitStatus(unix_wait_status(256))
```

Closes rust-lang#74832

…arth

Rollup of 14 pull requests

Successful merges:

 - rust-lang#86434 (Add `Ipv6Addr::is_benchmarking`)
 - rust-lang#86828 (const fn for option copied, take & replace)
 - rust-lang#87679 (BTree: refine some comments)
 - rust-lang#87910 (Mark unsafe methods NonZero*::unchecked_(add|mul) as const.)
 - rust-lang#88286 (Remove unnecessary unsafe block in `process_unix`)
 - rust-lang#88305 (Manual Debug for Unix ExitCode ExitStatus ExitStatusError)
 - rust-lang#88353 (Partially stabilize `array_methods`)
 - rust-lang#88370 (Add missing `# Panics` section to `Vec` method)
 - rust-lang#88481 (Remove some feature gates)
 - rust-lang#89138 (Fix link in Ipv6Addr::to_ipv4 docs)
 - rust-lang#89401 (Add truncate note to Vec::resize)
 - rust-lang#89467 (Fix typos in rustdoc/lints)
 - rust-lang#89472 (Only register `WSACleanup` if `WSAStartup` is actually ever called)
 - rust-lang#89505 (Add regression test for spurious const error with NLL)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup