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/computers/modular_computer.dm

@@ -271,6 +271,7 @@
 
 	data["PC_stationtime"] = worldtime2text()
 	data["PC_hasheader"] = 1
+	data["PC_showexitprogram"] = active_program ? 1 : 0 // Hides "Exit Program" button on mainscreen
 	return data