Form validator uses old data

When changing the cache dir in boost the boost_cache_directory() function is called from boost_admin_boost_performance_page_validate() to obtain the cache dir. Since this value has hot been influenced by the form submit the next test to verify if the cache directory exists fails in cases where the previously set cache directory does not exist.

An experimental patch is attached does a mini implementation of boost_cache_directory() on the spot.

It might become ugly code when $host, $subdir are also included here... maybe boost_cache_directory() could accept $boost_root_cache_dir and $boost_normal_dir as an optional overwrite parameter?