Improvements to NTNet and file mechanics

- Small adjustments to NTNet behaviour.
- Adds "undeletable" and "unmovable" flags for computer files. Intended to use for ROM-style programs that are necessary for PC to work.
- Programs now know NTNet status and can act differently depending on this.

code/modules/modular_computers/NTNet/NTNet.dm

@@ -16,6 +16,7 @@ var/global/datum/ntnet/ntnet_global = new()
 		ntnet_global = src // There can be only one.
 	for(var/obj/machinery/ntnet_relay/R in machines)
 		relays.Add(R)
+		R.NTNet = src
 
 /datum/ntnet/proc/check_function(var/specific_action = 0)
 	if(!relays || !relays.len) // No relays found. NTNet is down

code/modules/modular_computers/NTNet/NTNet_relay.dm

@@ -5,6 +5,9 @@
 	use_power = 1
 	idle_power_usage = 20000 //20kW, apropriate for machine that keeps massive cross-Zlevel wireless network operational.
 	icon_state = "bus"
+	anchored = 1
+	density = 1
+	var/NTNet = null // This is mostly for backwards reference and to allow varedit modifications from ingame.
 
 // TODO: Implement more logic here. For now it's only a placeholder.
 /obj/machinery/ntnet_relay/proc/is_operational()
@@ -15,7 +18,11 @@
 /obj/machinery/ntnet_relay/New()
 	if(ntnet_global)
 		ntnet_global.relays.Add(src)
+		NTNet = ntnet_global
+	..()
 
 /obj/machinery/ntnet_relay/Destroy()
 	if(ntnet_global)
-		ntnet_global.relays.Remove(src)
+		ntnet_global.relays.Remove(src)
+		NTNet = null
+	..()