du: handle --files0-from=- with piped in -

[cakebaker]

This PR handles the scenario in which --files0-from=- is specified and - is piped in. It should make the test du/files0-from.pl pass again.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[cakebaker]

Hm, for some reason the GNU test still fails, also locally :|

[cakebaker]

Ok, I think I figured out the issue: one of our string replacements in build-gnu.sh became outdated and worked only partially.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/du/files0-from is no longer failing!