'No Negative' function in D7 port

There is no D7 port of userpoints_no_negative, as that functionality is now implemented via Rules. However, the whole raison d' etre of userpoints_evaporate is to bulk manage points rather than run a rule for each up to millions of users, so it doesn't use userpoints API calls to assign points at all.

To stop users getting a negative points total, userpoints_evaporate should have a setting to control this per category.