du,ls: don’t modify getenv strings

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.