Cleanup can_fire(), projectile flag

Removes can_fire() and moves some of it's functionality into
get_next_projectile(), which was renamed to consume_next_projectile() to
indicate the possibility of side-effects.
Also renamed flag var to check_armour

code/modules/projectiles/projectile/change.dm

@@ -4,7 +4,7 @@
 	damage = 0
 	damage_type = BURN
 	nodamage = 1
-	flag = "energy"
+	check_armour = "energy"
 
 	on_hit(var/atom/change)
 		wabbajack(change)