* DiskCache: multiple fixes; support isWritable() for cache entries, set content-disposition for send()
* public/cached_url: allow selecting files from sub-caches other than images
* plugins/Cache_Starred_Images: rework to use DiskCache, can be enabled per-user, properly handles article enclosures, etc
* support various logging levels per-message
* remove hacks like debug_suppress, DAEMON_EXTENDED_DEBUG, etc
* _debug() is kept as a compatibility shim for plugins
implement a hook (HOOK_SEND_LOCAL_FILE) which plugins may use to send files
via httpd-specific implementation to increase performance typically on larger files
* add CACHE_MAX_DAYS as a tunable generic expiry interval for various cached files
* add some comments to tunables in functions.php
* rename _MIN_CACHE_FILE_SIZE to MIN_CACHE_FILE_SIZE
* respect MIN_CACHE_FILE_SIZE setting in a few more places where content is cached
Andrew Dolgov