Make Apache run our handler hook earlier.

The mod_auth_mellon hook is designed to handle requests to
MellonEndpointPath rather than looking at r->handler.

If we are unlucky, a different handler hook may look at r->handler,
and decide that the request is meant for it instead of mod_auth_mellon.

This patch makes the mod_auth_mellon hook run before most other hooks.

A workaround is to add something like this to the Apache configuration:

<Location /mellon>
  SetHandler mod_auth_mellon
</Location

mod_auth_mellon.c

@@ -188,7 +188,17 @@ static void register_hooks(apr_pool_t *p)
     ap_hook_check_user_id(am_check_uid, NULL, NULL, APR_HOOK_MIDDLE);
     ap_hook_post_config(am_global_init, NULL, NULL, APR_HOOK_MIDDLE);
     ap_hook_child_init(am_child_init, NULL, NULL, APR_HOOK_MIDDLE);
-    ap_hook_handler(am_handler, NULL, NULL, APR_HOOK_MIDDLE);
+
+    /* Add the hook to handle requests to the mod_auth_mellon endpoint.
+     *
+     * This is APR_HOOK_FIRST because we do not expect nor require users
+     * to add a SetHandler option for the endpoint. Instead, simply
+     * setting MellonEndpointPath should be enough.
+     *
+     * Therefore this hook must run before any handler that may check
+     * r->handler and decide that it is the only handler for this URL.
+     */
+    ap_hook_handler(am_handler, NULL, NULL, APR_HOOK_FIRST);
     return;
 }