Rollup merge of #72963 - poliorcetics:cstring-from-raw, r=dtolnay

Cstring `from_raw` and `into_raw` safety precisions

Fixes #48525.
Fixes #68456.

This issue had two points:

- The one about `from_raw` has been addressed (I hope).
- The other one, about `into_raw`, has only been partially fixed.

About `into_raw`: the idea was to:

> steer users away from using the pattern of CString::{into_raw,from_raw} when interfacing with C APIs that may change the effective length of the string by writing interior NULs or erasing the final NUL

I tried making a `Vec<c_char>` like suggested but my current solution feels very unsafe and *hacky* to me (most notably the type cast), I included it here to make it available for discussion:

```rust
fn main() {
    use std::os::raw::c_char;

    let v = String::from("abc")
        .bytes()
        // From u8 to i8,
        // I feel like it will be a problem for values of u8 > 255
        .map(|c| c as c_char)
        .collect::<Vec<_>>();

    dbg!(v);
}
```

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