Add `pure`/`nothrow`/`@nogc` attributes to `std.socket`.

Also add `in` parameter storage class to `validHostent` function.
Also add attributes to `std.c.windows.winsock`.
Make `getnameinfo`, `getaddrinfo`, and `freeaddrinfo` pointers `immutable`.
Add `@safe`/`@trusted`/`@system` attributes to `std.socket`.
[Fix][DDoc] Add missing third `/` to DDoc comment.
[Fix] Fix `SocketSet.count` return type.
Merge pull request #2316 from denis-sh/add-attributes-to-`std.socket`

Add attributes to `std.socket`