Preparing to remove the bundled PHPTAL and let the PHPTAL skins (if enabled) work...

Preparing to remove the bundled PHPTAL and let the PHPTAL skins (if enabled) work with an externally-installed PEAR+PHPTAL.
$wgUsePHPTal setting no longer exists; if a PHPTAL-based skin is used it will attempt to load PEAR+PHPTAL and gracefully disable itself if it can't.
Todo: get the list of skins in preferences to reflect this kind of disabling, so unselectable skins don't need to be moved into disabled subdir to be hidden.