puzzle_copy: Copy the solution list as well

puzzle_copy: Copy the solution list as well

Otherwise, the new puzzle will be pointing at the old puzzle's
solution list, (and the code that looks to see if the solution list
has changed won't notice any change).

This fixes things so that using "/solved" to add a second (or third,
...) to a puzzle will now update the channel's topic, which wasn't
happening before.