plugins: add error returns to plugins_configure()

These are not really errors.  They indicate a possible issue
but should not error.  An unknown hook may be normal given
that the plugin owning the hook is not installed.

We also don't consider hooks to return errors in a meaningful
way as they are meant as machinery to run a reconfiguration
which can register its own errors elsewhere in the system.

But this can give the user running manual queries a useful hint.

src/etc/inc/plugins.inc

@@ -262,6 +262,7 @@ function plugins_configure($hook, $verbose = false, $args = [])
 {
     $logargs = [];
     $glob = '*';
+    $ret = 1; /* no hook executed */
 
     array_unshift($args, $verbose);
 
@@ -318,6 +319,7 @@ function plugins_configure($hook, $verbose = false, $args = [])
                             $argf,
                             implode(',', array_slice($logargs, 0, $argc))
                         ), LOG_DEBUG);
+                        $ret = 0;
                         try {
                             call_user_func_array($argf, array_slice($args, 0, $argc));
                         } catch (\Error $e) {
@@ -328,6 +330,8 @@ function plugins_configure($hook, $verbose = false, $args = [])
             }
         }
     }
+
+    return $ret;
 }
 
 function plugins_run($hook, $args = [])

src/sbin/pluginctl

@@ -344,7 +344,11 @@ if (isset($opts['h'])) {
     /* second argument is hook */
     $hook = array_shift($args);
     /* other arguments are passed as is */
-    plugins_configure($hook, !isset($opts['q']), $args);
+    $ret_int = plugins_configure($hook, !isset($opts['q']), $args);
+    /* cannot taint $ret in this case */
+    if ($ret_int && !isset($opts['q'])) {
+        echo "The configuration hook `$hook' is not known to the system." . PHP_EOL;
+    }
 }
 
 exit($ret);