Change apc_drush_flush() to use two parameters instead of one

apc_drush_flush() uses a single parameter, an array with two values. The code would be much simpler if it used two parameters, the last of which has array() as default value.

This requires changing also the code used for apc_xmlrpc().

Since apc_drush_flush() is changed, I would also rename it to apc_clear_cache() and change its method name to apc.clear_cache.