[boost] [smart_ptr] small step approach

Hi, since passing or not passing the deleter to sp_accept_owner seems controversial, I'd like to try another approach: Smaller patches/steps. The attached patch is a minimalistic version of my previous patches. It doesn't change sp_accept_owner's current design. With this patch, I'd be able to implement my own, optimized version of enable_shared_from_this, which is otherwise not possible. Do you see any problem with this patch or with the approach in general? Regards, Daniel