Ability to use Drupal's watchdog for for CiviCRM error messages

Currently, CiviCRM writes it's debugging messages to a text file on the system, which is perhaps not the easiest thing to monitor for some people. The attached patch introduces a new settings on the /civicrm/admin/setting/debug page, which allows Drupal users to also channel CiviCRM's error/debugging messages through the Drupal watchdog. The option will only appear on the debugging settings page if Drupal is being used as the framework. If enabled, messages are written to the druapl watchdog with the WATCHDOG_DEBUG level. It would be nice to be able to use different error levels for different types of messages (eg. fatal = WATCHDOG_ERROR), but I'll leave that as an extension project