Added 'clearall' argument to /dsentry remove.

also added some more comments, and adjustments to eliminate errors from dsentry.targets being empty.

It was actually this last bit that started these changes off. As I don't want my bossfight npc's running off and attacking mobs, only players with the right flags.
This isn't even a valid tag. And irrelevant, if the entity doesn't have a location, errors /should/ show.