split: implement outputting kth chunk of file

Implement `-n l/k/N` option, where the `k`th chunk of the input file
is written to stdout. For example,

    $ seq -w 0 99 > f; split -n l/3/10 f
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29

tests/by-util/test_split.rs

@@ -587,3 +587,11 @@ fn test_lines() {
     assert_eq!(file_read("xaa"), "1\n2\n3\n");
     assert_eq!(file_read("xab"), "4\n5\n");
 }
+
+#[test]
+fn test_lines_kth() {
+    new_ucmd!()
+        .args(&["-n", "l/3/10", "onehundredlines.txt"])
+        .succeeds()
+        .stdout_only("20\n21\n22\n23\n24\n25\n26\n27\n28\n29\n");
+}