sync: fsync() file operands when no -d/-f flags (GNU MODE_FILE compat)

[lostallmymoney]

With file operands and no --data/-d or --file-system/-f flag, GNU sync enters MODE_FILE and fsync()s each operand. uutils instead falls through to a global sync(), silently ignoring the operands and blocking on every filesystem behind initramfs/dracut. Add an fsync(2) path (Linux/Android) mirroring do_fdatasync, and a regression test using /proc/self/mem which rejects fsync() with EINVAL (a global sync() would wrongly exit 0).

Fixes: #14591

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/sync. tests/misc/sync is passing on 'main'. Maybe you have to rebase?
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/bounded-memory is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[lostallmymoney]

I'm not an expert in github testing. Is the test failing because it's succeeding or is it really a lack of permissions ahah
The test needs to fail to succeed, to be a regression-prevention-test, idk if that's usually the way things go.