dd: refactor stdio handling

[xtqqczze]

• Introduce a StdinRaw wrapper backed by ManuallyDrop<File> to prevent the stdin file descriptor from being closed on drop, which could otherwise lead to undefined behavior.
• Introduce a StdoutRaw wrapper to avoid unnecessarily cloning the file descriptor as much as possible.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/pr/bounded-memory is now passing!