[boost] [smart_ptr] refactored enable_shared_from_this

I refactored enable_shared_from_this in order to remove the _owner flag. To test it, you need a small and straight-forward addition to detail::weak_count: It needs .empty(), similar to detail::shared_count. The attached version also hides all internal stuff from the public interface, using 'friend' - maybe some compiler would need a workaround for that. If someone objects to this (maybe because it might cause trouble with some compilers) I could leave this part out or fix it later where it breaks. The good thing of this is, that we no longer need _internal_-prefixes. OK to commit? Regards, Daniel