Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/ralouphie/getallheaders/src/getallheaders.php

@@ -0,0 +1,46 @@
+<?php
+
+if (!function_exists('getallheaders')) {
+
+    /**
+     * Get all HTTP header key/values as an associative array for the current request.
+     *
+     * @return string[string] The HTTP header key/value pairs.
+     */
+    function getallheaders()
+    {
+        $headers = array();
+
+        $copy_server = array(
+            'CONTENT_TYPE'   => 'Content-Type',
+            'CONTENT_LENGTH' => 'Content-Length',
+            'CONTENT_MD5'    => 'Content-Md5',
+        );
+
+        foreach ($_SERVER as $key => $value) {
+            if (substr($key, 0, 5) === 'HTTP_') {
+                $key = substr($key, 5);
+                if (!isset($copy_server[$key]) || !isset($_SERVER[$key])) {
+                    $key = str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', $key))));
+                    $headers[$key] = $value;
+                }
+            } elseif (isset($copy_server[$key])) {
+                $headers[$copy_server[$key]] = $value;
+            }
+        }
+
+        if (!isset($headers['Authorization'])) {
+            if (isset($_SERVER['REDIRECT_HTTP_AUTHORIZATION'])) {
+                $headers['Authorization'] = $_SERVER['REDIRECT_HTTP_AUTHORIZATION'];
+            } elseif (isset($_SERVER['PHP_AUTH_USER'])) {
+                $basic_pass = isset($_SERVER['PHP_AUTH_PW']) ? $_SERVER['PHP_AUTH_PW'] : '';
+                $headers['Authorization'] = 'Basic ' . base64_encode($_SERVER['PHP_AUTH_USER'] . ':' . $basic_pass);
+            } elseif (isset($_SERVER['PHP_AUTH_DIGEST'])) {
+                $headers['Authorization'] = $_SERVER['PHP_AUTH_DIGEST'];
+            }
+        }
+
+        return $headers;
+    }
+
+}