NTNet P2P Transfer client

- Adds simple NTNet P2P transfer client, that allows users to send files via NTNet.
- This client can act either as actual client (downloading files), or as a server, providing one file at time.
- This may (not guaranteed) be expanded in future, to allow sharing of multiple files at once, possibly even having some sort of dedicated server architecture
- Transfer speed is calculated as lower value of connection speed of both devices (weakest link in the chain)
- Server may be protected using rudimentary password that, if set, must be entered to download the file.

code/modules/modular_computers/file_system/computer_file.dm

@@ -1,3 +1,5 @@
+var/global/file_uid = 0
+
 /datum/computer_file/
 	var/filename = "NewFile" 								// Placeholder. No spacebars
 	var/filetype = "XXX" 									// File full names are [filename].[filetype] so like NewFile.XXX in this case
@@ -5,6 +7,12 @@
 	var/obj/item/weapon/computer_hardware/hard_drive/holder	// Holder that contains this file.
 	var/unsendable = 0										// Whether the file may be sent to someone via NTNet transfer or other means.
 	var/undeletable = 0										// Whether the file may be deleted. Setting to 1 prevents deletion/renaming/etc.
+	var/uid													// UID of this file
+
+/datum/computer_file/New()
+	..()
+	uid = file_uid
+	file_uid++
 
 /datum/computer_file/Destroy()
 	if(!holder)