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.
The errors here may have been due to the previous one, but having the definition filled by something that wasn't a valid location generated errors from the walk command.