cmd/cli: split-route SRV record to OS resolver

Since SRV record is mostly useful in AD environment. Even in non-AD one,
the OS resolver could still resolve the query for external services.

Users who want special treatment can still specify domain rules to
forward requests to ControlD upstreams explicitly.

cmd/cli/dns_proxy_test.go

@@ -414,6 +414,26 @@ func Test_isPrivatePtrLookup(t *testing.T) {
 	}
 }
 
+func Test_isSrvLookup(t *testing.T) {
+	tests := []struct {
+		name        string
+		msg         *dns.Msg
+		isSrvLookup bool
+	}{
+		{"SRV", newDnsMsgWithHostname("foo", dns.TypeSRV), true},
+		{"Not SRV", newDnsMsgWithHostname("foo", dns.TypeNone), false},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if got := isSrvLookup(tc.msg); tc.isSrvLookup != got {
+				t.Errorf("unexpected result, want: %v, got: %v", tc.isSrvLookup, got)
+			}
+		})
+	}
+}
+
 func Test_isWanClient(t *testing.T) {
 	tests := []struct {
 		name        string