shred: truncate file to zero before unlinking

[gabrielhnf]

Fixes #12287

When -u is given, shred now truncates the file to zero before attempting to unlink it. This ensures the file's allocated blocks are freed even when the parent directory is not writable (which would prevent unlinking).

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tty-eof. tests/misc/tty-eof is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!