Add attributes to `std.socket`

`std.socket` now has `@safe` public API.

Also added few fixes.
> There is no phobos policy requiring or encouraging in on value arguments that I'm aware of, and it honestly seems like a completely useless thing to do in most cases. Some people have advocated that in the past, but that doesn't make it more than some people's style preference.

> IMO the fact that it adds three characters is enough to tip the scale.

I agree.