Added cron shell disable feature

  New feature allow admin to decide if user can run any commands or
only schedule url loads. In a hosting with random clients enabling
users to run shell commands is highly discouraged.
  Database schema is modified, updatelib modified accordingly.
  Defaults to Off.
Better working solution for cron shell access
Even better Cron shell ACL

 Put it together with Will (Shazar) :)
 Now unpriviledged user may run php and wget (without - options).