Changes to allow dSentries to guard npc's as well as players.

I haven't tested this extensively, but I have managed to setup one npc that is guarding me, and another npc that is guarding my guard.  When I move, the first npc follows me, and the second follows the first.  no errors apparent on console, even with debug on. (well one minor error complaining that the npc target is not a valid player, in the if command at the new line 507.)

I also updated a few .flag[] to .has_flag[] where it seemed appropriate.
I've added a comment in reply to your recent comment, that is now in in the 'outdated diffs' section that is the second from the bottom of the list that is 12hrs old when I write this. I'm adding this comment as it took me a while to find your comment, and wanted to save you the same trouble. =)