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.
It's probably time I confessed that I'm not a programmer, or even in IT in my day job.  Writing code (and maintaining an MC server) is entirely a hobby.  I mention this, as the differences between "player object" and "player" are a little lost on me.  Watching the debug output I was seeing dsentry.follow_target holding p@<myUUID> or n@<the_NPC's_ID>, or is this what you were saying?  Surely if we want to match this with 'player' or 'npc' we will need to add a .type in somewhere?

I'll make the changes you suggest, and maybe you can explain to me sometime why one way is better than the other? (for my own learning?)