Replace in with const scope for src/core/sys/posix

[JinShil]

About This PR

Followup to
#2676
dlang/phobos#7110
#2680
#2684
#2693
#2694
#2695
#2697
#2699

This one of several PRs I intend to submit, breaking up #2677 into multiple PRs.

This PR only addresses code in src/core/sys/posix/

Background

This PR is in support of dlang/dmd#10179

in as a parameter storage class is defined as scope const. However in has not yet
been properly implemented so its current implementation is equivalent to const. Properly
implementing in now will likely break code, so it is recommended to avoid using in, and
explicitly use const or scope const instead, until in is properly implemented.

The use of in as a parameter storage class is already discouraged in the documentation. See https://dlang.org/spec/function.html#parameters

[dlang-bot]

Thanks for your pull request and interest in making D better, @JinShil! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2699"