split: add support for -e argument

Add the `-e` flag, which indicates whether to elide (that is, remove)
empty files that would have been created by the `-n` option.

The `-n` command-line argument gives a specific number of chunks into
which the input files will be split. If the number of chunks is
greater than the number of bytes, then empty files will be created for
the excess chunks. But if `-e` is given, then empty files will not be
created.

For example, contrast

    $ printf 'a\n' > f && split -e -n 3 f && cat xaa xab xac
    a
    cat: xac: No such file or directory

with

    $ printf 'a\n' > f && split -n 3 f && cat xaa xab xac
    a

tests/by-util/test_split.rs

@@ -2,7 +2,7 @@
 //  *
 //  * For the full copyright and license information, please view the LICENSE
 //  * file that was distributed with this source code.
-// spell-checker:ignore xzaaa sixhundredfiftyonebytes ninetyonebytes asciilowercase fghij klmno pqrst uvwxyz fivelines twohundredfortyonebytes
+// spell-checker:ignore xzaaa sixhundredfiftyonebytes ninetyonebytes threebytes asciilowercase fghij klmno pqrst uvwxyz fivelines twohundredfortyonebytes
 extern crate rand;
 extern crate regex;
 
@@ -526,3 +526,29 @@ fn test_include_newlines() {
     at.open("xac").read_to_string(&mut s).unwrap();
     assert_eq!(s, "5\n");
 }
+
+#[test]
+fn test_allow_empty_files() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.args(&["-n", "4", "threebytes.txt"])
+        .succeeds()
+        .no_stdout()
+        .no_stderr();
+    assert_eq!(at.read("xaa"), "a");
+    assert_eq!(at.read("xab"), "b");
+    assert_eq!(at.read("xac"), "c");
+    assert_eq!(at.read("xad"), "");
+}
+
+#[test]
+fn test_elide_empty_files() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.args(&["-e", "-n", "4", "threebytes.txt"])
+        .succeeds()
+        .no_stdout()
+        .no_stderr();
+    assert_eq!(at.read("xaa"), "a");
+    assert_eq!(at.read("xab"), "b");
+    assert_eq!(at.read("xac"), "c");
+    assert!(!at.plus("xad").exists());
+}