ls: add --null option (Bug#49716)

* NEWS, doc/coreutils.texi (General output formatting):
* src/ls.c (usage): Document this.
* src/ls.c (NULL_OPTION): New constant.
(long_options): Add --null.
(eolbyte): New static var.
(dired_dump_obstack, main, print_dir, print_current_files)
(print_many_per_line, print_horizontal, print_with_separator):
Output eolbyte instead of '\n'.
(decode_switches): Decode --null.
* tests/ls/null-option.sh: New file.
* tests/local.mk (all_tests): Add it.

tests/ls/null-option.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Verify behavior of ls --null.
+
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ ls
+
+mkdir dir && touch dir/a dir/b dir/cc || framework_failure_
+
+LC_ALL=C ls --null dir >out || fail=1
+tr '\n' '\0' <<EOF >exp
+a
+b
+cc
+EOF
+
+compare exp out || fail=1
+
+Exit $fail