Define wmi_container::operator=

If you are doing a deep copy in the copy constructor, you better
also do so in the assignment operator. (And this assignment operator
is used in carryover_info::transfer_from.)
Make wmi_container responsible for cleaning up its memory.

This looks like it was a memory leak, at least when carryover_info
was constructed from a config. Regardless, the new method is much
more robust, heading off future memory leaks.
Better use of "explicit" for constructors.