ls: support --time=creation to show/sort birth time

* src/ls.c (usage): Reorganize help for --time,
and add description for --time=birth.
(do_statx): Store btime in mtime if available.
(get_stat_btime): A new function to read the creation time
from the appropriate stat structure member.
(cmp_btime): A new function to compare birth time.
(print_long_format): Output '?' when birth time unavailable.
* doc/coreutils.texi: Document --time={birth,creation}.
* tests/local.mk: Reference the new test.
* tests/ls/birthtime.sh: Add a new test.
* NEWS: Mention the new feature.

tests/ls/birthtime.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+# ensure that ls attempts birthtime access
+
+# Copyright (C) 2020 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
+
+# ls should not fail, even if birth time not available
+touch a || framework_failure_
+ls --time=birth -l a || fail=1
+ls --time=creation -t a || fail=1
+
+Exit $fail