Issue 2368: tasks should be interruptable.  The four schedulable task (import-ldif, export-ldif, backup, and restore) can now be interrupted for purposes of cancellation.  The manage-tasks utility now allows the user to cancel any one of these tasks if they are currently running.  If interrupted while executing, the tasks try to break out of their work loop as soon as possible and return a 'stopped by administrator' status.  Both the backup and export-ldif tasks perform some cleanup (removing the abandoned backup or exported LDIF file) if they are cancelled.