Sort tempfile logic opens up extra files

As part of investigating this issue, found that we're using extra file descriptors to mange our tempfile logic for batched-sort operations. This is a problem because some of the GNU compatibility tests run with ulimit enforcing strict limits on the number of files that sort can open.

Looking at the open files when starting a batched sort, I see...

$ ls -l /proc/1778302/fd
total 0
lrwx------ 1 karl karl 64 Dec  8 19:43 0 -> /dev/pts/3
l-wx------ 1 karl karl 64 Dec  8 19:43 1 -> /home/karl/Rust/gnu/tests/sort/sort-merge-fdlimit.log
l-wx------ 1 karl karl 64 Dec  8 19:43 2 -> /home/karl/Rust/gnu/gt-sort-merge-fdlimit.sh.A1OJ/err/merge-random-err
lr-x------ 1 karl karl 64 Dec  8 19:43 3 -> 'pipe:[3021894]'
l-wx------ 1 karl karl 64 Dec  8 19:43 4 -> 'pipe:[3021894]'

It's the last two of these that are problematic. These two pipes are created when we set up the Tempfile logic...

    fn init_tmp_dir(&mut self) -> UResult<()> {
<snip>
        let path = self.temp_dir.as_ref().unwrap().path().to_owned();
        let lock = self.lock.clone();
// *** The line below will open the two pipes.
        ctrlc::set_handler(move || {
            // Take the lock so that `next_file_path` returns no new file path,
            // and the program doesn't terminate before the handler has finished
            let _lock = lock.lock().unwrap();
            if let Err(e) = remove_tmp_dir(&path) {
                show_error!("failed to delete temporary directory: {}", e);
            }
            std::process::exit(2)
        })
        .map_err(|e| USimpleError::new(2, format!("failed to set up signal handler: {e}")))
    }

To fix this, I propose to just simplify this logic and just remove the Tempdir altogether. Just put any temporary files directly into /tmp (which is what GNU-sort does) and have the OS do the cleanup automatically rather than create a dedicated folder for them. We can then remove this code above that uses the ctrlc crate.

I don't know the history on this, so maybe there's a good reason to use a separate folder - If so, please let me know!

Start at sort's init_tmp_dir logic and trace how Tempfile, the temporary directory, and the ctrlc handler are used during batched sorts. Run the GNU compatibility tests for sort-merge-fdlimit and inspect open descriptors while sorting. Done means the extra pipes are no longer opened and the fd-limit tests still pass.