Replace magic intent words with defines, grab fixes

This commit does the following:
 - Replaces all instances of magic intent words, like "harm" and
 "help" with defines, I_HELP, I_DISARM, I_GRAB, I_HARM

 - Fixes a few manual grab instances left over in attack_alien, they
 will now used grabbedby()

code/modules/projectiles/gun.dm

@@ -295,7 +295,7 @@
 
 	if (src.process_chambered())
 		//Point blank shooting if on harm intent or target we were targeting.
-		if(user.a_intent == "harm")
+		if(user.a_intent == I_HARM)
 			user.visible_message("\red <b> \The [user] fires \the [src] point blank at [M]!</b>")
 			if(istype(in_chamber)) in_chamber.damage *= 1.3
 			Fire(M,user,0,0,1)

code/modules/projectiles/projectile/magic.dm

@@ -232,7 +232,7 @@ proc/wabbajack(mob/living/M)
 			M.attack_log += text("\[[time_stamp()]\] <font color='orange'>[M.real_name] ([M.ckey]) became [new_mob.real_name].</font>")
 			new_mob.attack_log = M.attack_log
 
-			new_mob.a_intent = "harm"
+			new_mob.a_intent = I_HARM
 			if(M.mind)
 				M.mind.transfer_to(new_mob)
 			else