caddyhttp: host labels placeholders endianness from right->left

https://caddy.community/t/labeln-placeholder-endian-issue/5366

(I thought we had this before but it must have gotten lost somewhere)

modules/caddyhttp/replacer.go

@@ -146,14 +146,18 @@ func addHTTPVarsToReplacer(repl caddy.Replacer, req *http.Request, w http.Respon
 				if err != nil {
 					return "", false
 				}
-				hostLabels := strings.Split(req.Host, ".")
+				reqHost, _, err := net.SplitHostPort(req.Host)
+				if err != nil {
+					reqHost = req.Host // OK; assume there was no port
+				}
+				hostLabels := strings.Split(reqHost, ".")
 				if idx < 0 {
 					return "", false
 				}
-				if idx >= len(hostLabels) {
+				if idx > len(hostLabels) {
 					return "", true
 				}
-				return hostLabels[idx], true
+				return hostLabels[len(hostLabels)-idx-1], true
 			}
 
 			// path parts

modules/caddyhttp/replacer_test.go

@@ -61,6 +61,14 @@ func TestHTTPVarReplacement(t *testing.T) {
 			input:  "{http.request.remote.port}",
 			expect: "1234",
 		},
+		{
+			input:  "{http.request.host.labels.0}",
+			expect: "com",
+		},
+		{
+			input:  "{http.request.host.labels.1}",
+			expect: "example",
+		},
 	} {
 		actual := repl.ReplaceAll(tc.input, "<empty>")
 		if actual != tc.expect {