Migrate ACLs syntax to new Tailscale format

Implements #617.

Tailscale has changed the format of their ACLs to use a more firewall-y terms ("users" & "ports" -> "src" & "dst"). They have also started using all-lowercase tags. This PR applies these changes.

tests/acls/acl_policy_basic_wildcards.yaml

@@ -1,10 +1,10 @@
 ---
-Hosts:
+hosts:
   host-1: 100.100.100.100/32
   subnet-1: 100.100.101.100/24
-ACLs:
-  - Action: accept
-    Users:
+acls:
+  - action: accept
+    src:
       - "*"
-    Ports:
+    dst:
       - host-1:*