head: add abstractions for "all but last n lines"

Add some abstractions to simplify the `rbuf_but_last_n_lines()`
function, which implements the "take all but the last `n` lines"
functionality of the `head` program. This commit adds

- `RingBuffer`, a fixed-size ring buffer,
- `ZLines`, an iterator over zero-terminated "lines",
- `TakeAllBut`, an iterator over all but the last `n` elements of an
  iterator.

These three together make the implementation of
`rbuf_but_last_n_lines()` concise.

tests/by-util/test_head.rs

@@ -129,6 +129,15 @@ fn test_zero_terminated_syntax_2() {
         .stdout_is("x\0y");
 }
 
+#[test]
+fn test_zero_terminated_negative_lines() {
+    new_ucmd!()
+        .args(&["-z", "-n", "-1"])
+        .pipe_in("x\0y\0z\0")
+        .run()
+        .stdout_is("x\0y\0");
+}
+
 #[test]
 fn test_negative_byte_syntax() {
     new_ucmd!()