uucore/refactor ~ fix cargo clippy complaints (*allow* temporary_cstring_as_ptr; added FixME note)

* otherwise, unsure how to fix this; the following change

```rust
let res = unsafe { utmpxname(CString::new(f).unwrap().as_ptr()) };
```

to

```rust
let string_ptr = CString::new(f).unwrap().as_ptr();
let res = unsafe { utmpxname(string_ptr) };
```

causes three `who` tests (test_who::test_all, test_who::test_boot, and test_who::test_login)
to fail by generating no output.

src/uucore/src/lib/features/utmpx.rs

@@ -242,6 +242,8 @@ impl UtmpxIter {
     ///
     /// If not set, default record file will be used(file path depends on the target OS)
     pub fn read_from(self, f: &str) -> Self {
+        // FixME: discuss and revise a rewrite which is correct and satisfies clippy/rustc
+        #[allow(clippy::temporary_cstring_as_ptr)]
         let res = unsafe { utmpxname(CString::new(f).unwrap().as_ptr()) };
         if res != 0 {
             println!("Warning: {}", IOError::last_os_error());