du: --apparent counts only symlinks and regular

Problem reported by Christoph Anton Mitterer (Bug#61884).
* src/du.c (process_file): When counting apparent sizes, count
only usable st_size members.
* tests/du/apparent.sh: New file.
* tests/local.mk (all_root_tests): Add it.

tests/du/apparent.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Exercise du's --apparent-size option.
+
+# Copyright 2023 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_ du
+
+mkdir -p d || framework_failure_
+for f in $(seq 100); do
+  echo foo >d/$f || framework_failure_
+done
+
+du -b d/* >separate || fail=1
+du -b d   >together || fail=1
+separate_sum=$($AWK '{sum+=$1}END{print sum}' separate) || framework_failure_
+together_sum=$($AWK '{sum+=$1}END{print sum}' together) || framework_failure_
+test $separate_sum -eq $together_sum || fail=1
+
+Exit $fail