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().

test/intrusive_ptr_test.cpp

@@ -330,6 +330,13 @@ void test()
 
         BOOST_TEST(get_pointer(px) == px.get());
     }
+
+    {
+        boost::intrusive_ptr<X> px(new X);
+        X* detached = px.detach();
+        BOOST_TEST(detached->use_count() == 1);
+        delete detached;
+    }
 }
 
 } // namespace n_access