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/pm/pm_func.php

@@ -118,6 +118,9 @@ class pmbox_manager
 			}
 			else
 			{
+				if (!isset($pm_info['inbox']) || !is_array($pm_info['inbox'])) $pm_info['inbox'] = [];
+				if (!isset($pm_info['outbox']) || !is_array($pm_info['outbox'])) $pm_info['outbox'] = [];
+
 				$pm_info['inbox']['limit'] = '';
 				$pm_info['outbox']['limit'] = '';
 				$pm_info['inbox']['filled'] = '';