process-util: introduce prctl_safe() and port everything over to it (#43006)

prctl() is an API full of pitfalls: it is variadic, and some interfaces
don't expect zero-initialization of excess arguments, and others do.
Moreover, the parameters are "long", and nonetheless we usually pass
"int" to them. If we are too dumb to call it properly, let's just not
call it directly anymore, but let's add a wrapper around it that makes
the function non-variadic and declares the right types. Then, let's port
over everything to it.

This is inspired by #42996, but we had issues with this many times,
before and looking at this PR one can see that we otherwise still are
having the issue at numerous other places.