Consistently use io error handlers in Unix shims

[RalfJung]

Unix shims return -1 and set the thread-local "last error" variable to indicate an error occurred. With #3923 and #3929, Miri has some nice helpers to help with that, but they are not being used everywhere yet. So this is about refactoring the existing code to use the new helpers.

To help, start by grepping for -1 and for this.eval_libc("E in the src/shims/unix folder. You will find a few patterns:

                    let efault = this.eval_libc("EFAULT");
                    this.set_last_error(efault)?;
                    this.write_int(-1, dest)?;

should become this.set_last_error_and_return(LibcError("EFAULT"), dest)?;.

            this.set_last_error(ErrorKind::PermissionDenied)?;
            return Ok(Scalar::from_i32(-1));

should become return this.set_last_error_and_return_i32(ErrorKind::PermissionDenied);.

Feel free to ask if you encounter other patterns you are unsure about. It's also okay for the first PR to only cover parts of this and leave the harder cases for a second round.

[noahmbright]

@rustbot claim

I'll make a PR after a first pass. I can add a second commit or just fixup depending on how that goes. Thanks for the instructions!

[RalfJung]

After #3941, grepping for this.eval_libc("E still shows 36 hits. Most of these can probably be using set_last_error(LibcError(...)) instead, so that might be a good next step.

If this happens to be near a "return -1", it can use the helpers for that directly.