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/power/antimatter/control.dm

@@ -127,7 +127,7 @@
 
 
 /obj/machinery/power/am_control_unit/bullet_act(var/obj/item/projectile/Proj)
-	if(Proj.flag != "bullet")
+	if(Proj.check_armour != "bullet")
 		stability -= Proj.force
 	return 0
 

code/modules/power/antimatter/shielding.dm

@@ -117,7 +117,7 @@ proc/cardinalrange(var/center)
 
 
 /obj/machinery/am_shielding/bullet_act(var/obj/item/projectile/Proj)
-	if(Proj.flag != "bullet")
+	if(Proj.check_armour != "bullet")
 		stability -= Proj.force/2
 	return 0
 

code/modules/power/rust/virtual_particle_catcher.dm

@@ -42,7 +42,7 @@
 		name = "collector [mysize] OFF"
 
 /obj/effect/rust_particle_catcher/bullet_act(var/obj/item/projectile/Proj)
-	if(Proj.flag != "bullet" && parent)
+	if(Proj.check_armour != "bullet" && parent)
 		parent.AddEnergy(Proj.damage * 20, 0, 1)
 		update_icon()
 	return 0