Another batch with a lot of internal updates, yet no visual changes to the
site:

- watchdog (rewrite):
   + the collected information provides more details and insights
     for post-mortem research
   + input limitation
- database abstraction layer:
   + mysql errors are now verbose and is no longer displayed in a
     browser - fixes a possible security risk
- admin.php:
   + updated watchdog page
   + fixed security flaw
- diary.php:
   + fixed nl2br problem
- themes:
   + fixed comment bug in all 3 themes.
- misc:
   + renamed some global variables for sake of consistency:
      $sitename  -->  $site_name
      $siteurl   -->  $site_url
   + added input check where (a) exploitable and (b) possible
   + added input size check
   + various small improvements
   + fixed various typoes

... and much, much more in fact.