gnt-job cancel: Confirmation and selection of jobs

New parameters, “--pending”, “--queued” and “--waiting”, are added to
select all jobs in the respective state. If one of those options is used
and “--force” is not given, the user is asked to confirm the operation.
Unit tests are included.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>