Restore support for "Override local DNS" (#2438)

Tailscale allows to override the local DNS settings of a node via
"Override local DNS" [1]. Restore this flag with the same config setting
name `dns.override_local_dns` but disable it by default to align it with
Tailscale's default behaviour.

Tested with Tailscale 1.80.2 and systemd-resolved on Debian 12.

With `dns.override_local_dns: false`:

```
Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: -DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa [snip]
```

With `dns.override_local_dns: true`:

```
Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~.
```

[1] https://tailscale.com/kb/1054/dns#override-local-dns

Fixes: #2256

hscontrol/types/testdata/base-domain-in-server-url.yaml

@@ -13,3 +13,4 @@ server_url: "https://server.derp.no"
 dns:
   magic_dns: true
   base_domain: derp.no
+  override_local_dns: false

hscontrol/types/testdata/base-domain-not-in-server-url.yaml

@@ -13,3 +13,4 @@ server_url: "https://derp.no"
 dns:
   magic_dns: true
   base_domain: clients.derp.no
+  override_local_dns: false

hscontrol/types/testdata/dns-override-true-error.yaml

@@ -0,0 +1,16 @@
+noise:
+  private_key_path: "private_key.pem"
+
+prefixes:
+  v6: fd7a:115c:a1e0::/48
+  v4: 100.64.0.0/10
+
+database:
+  type: sqlite3
+
+server_url: "https://server.derp.no"
+
+dns:
+  magic_dns: true
+  base_domain: derp.no
+  override_local_dns: true

hscontrol/types/testdata/dns-override-true.yaml

@@ -0,0 +1,20 @@
+noise:
+  private_key_path: "private_key.pem"
+
+prefixes:
+  v6: fd7a:115c:a1e0::/48
+  v4: 100.64.0.0/10
+
+database:
+  type: sqlite3
+
+server_url: "https://server.derp.no"
+
+dns:
+  magic_dns: true
+  base_domain: derp2.no
+  override_local_dns: true
+  nameservers:
+    global:
+      - 1.1.1.1
+      - 1.0.0.1

hscontrol/types/testdata/dns_full.yaml

@@ -7,6 +7,7 @@ dns:
   magic_dns: true
   base_domain: example.com
 
+  override_local_dns: false
   nameservers:
     global:
       - 1.1.1.1

hscontrol/types/testdata/dns_full_no_magic.yaml

@@ -7,6 +7,7 @@ dns:
   magic_dns: false
   base_domain: example.com
 
+  override_local_dns: false
   nameservers:
     global:
       - 1.1.1.1

hscontrol/types/testdata/policy-path-is-loaded.yaml

@@ -15,4 +15,6 @@ policy:
   type: file
   path: "/etc/policy.hujson"
 
-dns.magic_dns: false
+dns:
+  magic_dns: false
+  override_local_dns: false