Use querystring for HTTP caches

Improve HTTP compliancy: use query string instead of HTTP custom header.

README.md

@@ -121,17 +121,17 @@ Center page, which needs the Distfeed content. On the other hand the
 nightly cluster update procedure wants to install managed updates
 automatically for both the base Rocky Linux OS and NS8 applications.
 
-Web clients can send an optional `X-Repo-View` header, with value `latest`
-or `managed`. Any other value is considered like `latest`, but this
-behavior may change in the future.
+Web clients can send an optional `view` querystring parameter, with value
+`latest` or `managed`. Any other value is considered like `latest`, but
+this behavior may change in the future.
 
 Header codes:
 
 - `A` Authenticated
 - `U` Anonymous
 - `M` Managed repo view
 - `L` Latest repo view
-- `X` X-Repo-View header not sent
+- `X` querystring `view` parameter not present
 
 Request types:
 

imageroot/script/snapshot.php

@@ -73,7 +73,7 @@ function serve_from_snapshots() {
 }
 
 function main() {
-    $repo_view = isset($_SERVER['HTTP_X_REPO_VIEW']) ? $_SERVER['HTTP_X_REPO_VIEW'] : "unknown";
+    $repo_view = isset($_GET['view']) ? $_GET['view'] : "unknown";
     $username = isset($_SERVER['PHP_AUTH_USER']) ? $_SERVER['PHP_AUTH_USER'] : "";
     $password = isset($_SERVER['PHP_AUTH_PW']) ? $_SERVER['PHP_AUTH_PW'] : "";
 
@@ -94,8 +94,8 @@ function main() {
     //
 
     if($repo_view == "unknown") {
-        // Core <2.10 does not send the X-Repo-View header. We implement
-        // the initial update policy for backward compatibility.
+        // Core <2.10 does not send the querystring "view" parameter. We
+        // implement the initial update policy for backward compatibility.
         if($is_authenticated) {
             $prefix = serve_from_snapshots(); // AX => SS
         } else {