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/fixtures/split/threebytes.txt

@@ -0,0 +1 @@
+abc