This function handles the cache-related settings.

• Detects whether the various supported optimizers are installed based on the existence of an appropriate cache-storing function from each.
• Detects them in the following order: eAccelerator ('eaccelerator_put'), MMCache ('mmcache_put'), APC ('apc_store') and Zend Engine ('output_cache_put')
• Allows you to specify the memcache servers if you're using memcache
• Redirects back to itself and saves its own settings.