map bool -> map struct{} (#2386)

This clear out the remaining map[x]bool usage and moves the bool to an
empty struct.

Two note worthy other changes:

* EnableChaos in the server is now also exported to make it show up in
  the documentation.
* The auto plugin is left as is, because there the boolean is
  explicitaly set to false to signal 'to-be-deleted' and the key is left
  as-is.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/kubernetes/xfr_test.go

@@ -129,9 +129,9 @@ func TestKubernetesXFRNotAllowed(t *testing.T) {
 
 // difference shows what we're missing when comparing two RR slices
 func difference(testRRs []dns.RR, gotRRs []dns.RR) []dns.RR {
-	expectedRRs := map[string]bool{}
+	expectedRRs := map[string]struct{}{}
 	for _, rr := range testRRs {
-		expectedRRs[rr.String()] = true
+		expectedRRs[rr.String()] = struct{}{}
 	}
 
 	foundRRs := []dns.RR{}