[boost] [smart_ptr] refactored enable_shared_from_this

From: Daniel Frey (d.frey_at_[hidden])
Date: 2008-04-26 03:53:53

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