Rollup merge of #90430 - jkugelman:must-use-std-a-through-n, r=joshtriplett

Add #[must_use] to remaining std functions (A-N)

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is half of the remaining items from the `std` crate, from A-N.

I added these functions myself. Clippy predictably ignored the `mut` ones, but I don't know why the rest weren't flagged. Check them closely, please? Maybe I overlooked good reasons.

```rust
std::backtrace::Backtrace                                   const fn disabled() -> Backtrace;
std::backtrace::Backtrace<'a>                               fn frames(&'a self) -> &'a [BacktraceFrame];
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn key_mut(&mut self) -> &mut K;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_mut(&mut self) -> &mut V;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value(&mut self) -> (&K, &V);
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value_mut(&mut self) -> (&mut K, &mut V);
std::env                                                    fn var_os<K: AsRef<OsStr>>(key: K) -> Option<OsString>;
std::env                                                    fn split_paths<T: AsRef<OsStr> + ?Sized>(unparsed: &T) -> SplitPaths<'_>;
std::io::Error                                              fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)>;
```

Parent issue: #89692

r? `@joshtriplett`

library/std/src/fs.rs

@@ -374,6 +374,7 @@ impl File {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[unstable(feature = "with_options", issue = "65439")]
     pub fn with_options() -> OpenOptions {
         OpenOptions::new()
@@ -983,6 +984,7 @@ impl Metadata {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "file_type", since = "1.1.0")]
     pub fn file_type(&self) -> FileType {
         FileType(self.0.file_type())
@@ -1100,6 +1102,7 @@ impl Metadata {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn permissions(&self) -> Permissions {
         Permissions(self.0.perm())
@@ -1247,6 +1250,7 @@ impl Permissions {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use = "call `set_readonly` to modify the readonly flag"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn readonly(&self) -> bool {
         self.0.readonly()
@@ -1441,6 +1445,7 @@ impl DirEntry {
     /// ```
     ///
     /// The exact text, of course, depends on what files you have in `.`.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn path(&self) -> PathBuf {
         self.0.path()
@@ -1536,6 +1541,7 @@ impl DirEntry {
     ///     }
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "dir_entry_ext", since = "1.1.0")]
     pub fn file_name(&self) -> OsString {
         self.0.file_name()