httpserver: Raise error when adjusted site addresses clash at startup

See discussion on #2015 for how this situation was discovered. For a
Caddyfile like this:

	localhost {
		...
	}
	:2015 {
		...
	}

Running Caddy like this:

	caddy -host localhost

Produces two sites both defined as `localhost:2015` because the flag
changes the default host value to be `localhost`. This should be an
error since the sites are not distinct and it is confusing. It can also
cause issues with TLS handshakes loading the wrong cert, as the linked
discussion shows.

caddyhttp/httpserver/plugin_test.go

@@ -153,6 +153,23 @@ func TestInspectServerBlocksWithCustomDefaultPort(t *testing.T) {
 	}
 }
 
+// See discussion on PR #2015
+func TestInspectServerBlocksWithAdjustedAddress(t *testing.T) {
+	Port = DefaultPort
+	Host = "example.com"
+	filename := "Testfile"
+	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)
+	input := strings.NewReader("example.com {\n}\n:2015 {\n}")
+	sblocks, err := caddyfile.Parse(filename, input, nil)
+	if err != nil {
+		t.Fatalf("Expected no error setting up test, got: %v", err)
+	}
+	_, err = ctx.InspectServerBlocks(filename, sblocks)
+	if err == nil {
+		t.Fatalf("Expected an error because site definitions should overlap, got: %v", err)
+	}
+}
+
 func TestInspectServerBlocksCaseInsensitiveKey(t *testing.T) {
 	filename := "Testfile"
 	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)