du,ls: don’t modify getenv strings

Fix du and ls to conform to the POSIX getenv spec,
which says you can’t modify strings returned by getenv
unless you put the string there directly, or used putenv.
This portability bug was found by strict C23 checking
using qualifier-generic functions.
* bootstrap.conf (gnulib_modules): Add xmemdup0.  Sort.
* src/du.c (main):
* src/ls.c (decode_switches):
Don’t modify the string that getenv returns.
Instead, use xmemdup0 if needed, and include xmemdup0.h.

bootstrap.conf

@@ -69,8 +69,8 @@ gnulib_modules="
   crc-x86_64
   crypto/md5
   crypto/sha1
-  crypto/sha3
   crypto/sha256
+  crypto/sha3
   crypto/sha512
   crypto/sm3
   cycle-check
@@ -215,14 +215,14 @@ gnulib_modules="
   pipe2
   posix-shell
   posix_spawn
-  posix_spawnattr_destroy
-  posix_spawnattr_init
-  posix_spawnattr_setflags
-  posix_spawnattr_setsigdefault
   posix_spawn_file_actions_addclose
   posix_spawn_file_actions_adddup2
   posix_spawn_file_actions_destroy
   posix_spawn_file_actions_init
+  posix_spawnattr_destroy
+  posix_spawnattr_init
+  posix_spawnattr_setflags
+  posix_spawnattr_setsigdefault
   posix_spawnp
   posixtm
   posixver
@@ -325,6 +325,7 @@ gnulib_modules="
   xgetgroups
   xgethostname
   xmemcoll
+  xmemdup0
   xnanosleep
   xprintf
   xprintf-posix