sync: reduce use of unsafe and improve comments

[jtracey]

1. Safe wrappers exist for the unix side of things, so let's use those.
2. A function isn't unsafe just because it uses unsafe code, it's only unsafe if it can be invoked in a way that violates safety guarantees.
3. Safety comments shouldn't document why something is unsafe, they're for documenting either how to invoke something unsafe safely, or why the unsafe invocation is actually safe.

[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/timeout/timeout (fails in this run but passes in the 'main' branch)

[sylvestre]

excellent, thanks