sort: compatibility of human-numeric sort

Closes #1985.
This makes human-numeric sort follow the same algorithm as GNU's/FreeBSD's sort.
As documented by GNU in https://www.gnu.org/software/coreutils/manual/html_node/sort-invocation.html,
we first compare by sign, then by si unit and finally by the numeric value.

tests/by-util/test_sort.rs

@@ -456,10 +456,20 @@ fn test_human_block_sizes2() {
             .arg(human_numeric_sort_param)
             .pipe_in(input)
             .succeeds()
-            .stdout_only("-8T\n0.8M\n8981K\n21G\n909991M\n");
+            .stdout_only("-8T\n8981K\n0.8M\n909991M\n21G\n");
     }
 }
 
+#[test]
+fn test_human_numeric_zero_stable() {
+    let input = "0M\n0K\n-0K\n-P\n-0M\n";
+    new_ucmd!()
+        .arg("-hs")
+        .pipe_in(input)
+        .succeeds()
+        .stdout_only(input);
+}
+
 #[test]
 fn test_month_default2() {
     for month_sort_param in &["-M", "--month-sort", "--sort=month"] {

tests/fixtures/sort/human_block_sizes.expected

@@ -1,3 +1,4 @@
+0K
 K
 844K
 981K

tests/fixtures/sort/human_block_sizes.expected.debug

@@ -1,3 +1,6 @@
+0K
+__
+__
 K
 ^ no match for key
 _

tests/fixtures/sort/human_block_sizes.txt

@@ -9,4 +9,5 @@
 844K
 981K
 13M
-K
+K
+0K