Merge: More on Sockets and FinalizableOnce

Additionnal notes:
* `FinalizableOnce` will probably be the most common `Finalizable`, but it wouldn't cover all cases.
* Remove some unused features from `socket` to make it compatible with Android.
* Fix an illegal extern class by making it a pointer.
* Do not `exit 1` on socket error.

Pull-Request: #1066
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>