Fix Drupal 7 errors/warnings about count(), cache_get(), and each().

Upgrading PHP to 7.2 might break your Drupal 7.

Here are some error/warning examples and how to fix them.

1. PHP Fatal error: Uncaught Error: Call to undefined function cache_get() in /usr/share/drupal7/includes/module.inc: 750
   • replace ini_set('session.save_handler', 'user'); with session_set_save_handler(new SessionHandler()); in settings.php
   • https://www.drupal.org/project/drupal/issues/3026449
2. Warning: count(): Parameter must be an array or an object that implements Countable in theme_table() (line 1997 of /usr/share/drupal7/includes/theme.inc).
   • replace count() with !empty()
   • https://www.drupal.org/project/drupal/issues/2938567
3. Deprecated function: The each() function is deprecated. This message will be suppressed on further calls in menu_set_active_trail() (line 2404 of /usr/share/drupal7/includes/menu.inc).
   • deprecate exch()
   • https://www.drupal.org/project/drupal/issues/2925449