Add attributes to `std.socket`

`std.socket` now has `@safe` public API.

Also added few fixes.
> Phobos rules should reflect the reality of current D, not someone's idea of an ideal D.

And IMHO the reality is `in` was added with a desire it should be on every argument.

> So you only argue when you are unsure yourself?

Nop. It just hard and unpleasant to argue things you consider too obvious to even talk about.

> It's clearly not that obvious when Phobos doesn't have such a rule despite years of development and the participation of dozens of developers.

The only rule I see in Phobos is a formatting one. Otherwise the code-base leaves much to be desired.