[AltServer] Advertises unique server ID + writes to AltStore Info.plist

AltServer adds its own server ID to AltStore’s Info.plist when resigning. This allows AltStore to prefer the server it was installed with.

AltServer/Connections/UserDefaults+AltServer.swift

@@ -0,0 +1,29 @@
+//
+//  UserDefaults+AltServer.swift
+//  AltServer
+//
+//  Created by Riley Testut on 7/31/19.
+//  Copyright © 2019 Riley Testut. All rights reserved.
+//
+
+import Foundation
+
+extension UserDefaults
+{
+    var serverID: String? {
+        get {
+            return self.string(forKey: "serverID")
+        }
+        set {
+            self.set(newValue, forKey: "serverID")
+        }
+    }
+    
+    func registerDefaults()
+    {
+        if self.serverID == nil
+        {
+            self.serverID = UUID().uuidString
+        }
+    }
+}