Improve backtraces configuration.

The previous situation was confusing with various contradicting
(staments of) defaults, mixed with old style and new style
configuration.

Simplify by deciding that having backtraces is so
useful that it is the defaut, both old and new style configurations.
(It was already default on in the config template,
which seems 'canonical'.)

(cherry picked from commit 61453e96)

config-templates/config.php

@@ -237,9 +237,8 @@ $config = [
      * SAML messages will be logged, including plaintext versions of encrypted
      * messages.
      *
-     * - 'backtraces': this action controls the logging of error backtraces. If you
-     * want to log backtraces so that you can debug any possible errors happening in
-     * SimpleSAMLphp, enable this action (add it to the array or set it to true).
+     * - 'backtraces': this action controls the logging of error backtraces so you
+     * can debug any possible errors happening in SimpleSAMLphp.
      *
      * - 'validatexml': this action allows you to validate SAML documents against all
      * the relevant XML schemas. SAML 1.1 messages or SAML metadata parsed with

lib/SimpleSAML/Error/Exception.php

@@ -204,18 +204,9 @@ class Exception extends \Exception
      */
     protected function logBacktrace($level = Logger::DEBUG)
     {
-        // see if debugging is enabled for backtraces
-        $debug = Configuration::getInstance()->getArrayize('debug', ['backtraces' => false]);
-
-        if (
-            !(in_array('backtraces', $debug, true) // implicitly enabled
-            || (array_key_exists('backtraces', $debug)
-            && $debug['backtraces'] === true)
-            // explicitly set
-            // TODO: deprecate the old style and remove it in 2.0
-            || (array_key_exists(0, $debug)
-            && $debug[0] === true)) // old style 'debug' configuration option
-        ) {
+        // Do nothing if backtraces have been disabled in config.
+        $debug = Configuration::getInstance()->getArrayize('debug', ['backtraces' => true]);
+        if (array_key_exists('backtraces', $debug) && $debug['backtraces'] === false) {
             return;
         }