Fix all PHP 8.1 test failures

* `strftime()` has been replaced with a polyfill based on `DateTime`.
* Explicit type casts/assertions added where required by PHP 8.1
* `filter_var(…, FILTER_SANITIZE_STRING)` replaced with `strip_tags()`
  or HTML entity encoding of quotation marks, depending on a guess of
  what the intended "sanitization" was
* `http_build_query()` usage type mismatches fixed
* Removed usages of the `FILE_TEXT` constant
* To avoid breaking PHP 5.6 compatibility (function return types),
  `e_session_db` no longer implements `SessionHandlerInterface`.
  Instead, the alternative non-OOP invocation of
  `session_set_save_handler()` is used instead to apply the session
  handler.
* The shim for `strptime()` still calls the native function if available
  but now suppresses the deprecation warning.

* `e_db_pdo` explicitly asks for `PDO::ATTR_STRINGIFY_FETCHES` to
  maintain consistent behavior with past versions of PHP.
* `e_db_mysql` explicitly sets `mysqli_report(MYSQLI_REPORT_OFF)` to
  maintain consistent behavior with past versions of PHP.

* Removed pointless random number generator seed from `banner` plugin
* Workaround for `COUNT(*)` SQL query in
  `validatorClass::dbValidateArray()` without a proper API for avoiding
  SQL injection

e107_plugins/forum/forum_stats.php

@@ -65,6 +65,7 @@ class forumStats
 
 		$firstpost = $sql->select('forum_post', 'post_datestamp', 'post_datestamp > 0 ORDER BY post_datestamp ASC LIMIT 0,1', 'default');
 		$fp = $sql->fetch();
+		$fp = is_array($fp) ? $fp : array();
 
 		$open_ds = (int) varset($fp['post_datestamp']);
 		$open_date = $gen->convert_date($open_ds, 'long');