Add intrusive_ptr<>::detach()

This provides a way to escape from automatic reference counting, and taking
manual control of the reference.  Useful when interfacing to a C API that
expects a pointer with an elevated reference count.

Similar to std::unique_ptr<>::release().

include/boost/smart_ptr/intrusive_ptr.hpp

@@ -151,6 +151,13 @@ public:
         return px;
     }
 
+    T * detach() BOOST_NOEXCEPT
+    {
+        T * ret = px;
+        px = 0;
+        return ret;
+    }
+
     T & operator*() const
     {
         BOOST_ASSERT( px != 0 );