blockjobs: add ABORTING state

blockjobs: add ABORTING state

Add a new state ABORTING.

This makes transitions from normative states to error states explicit
in the STM, and serves as a disambiguation for which states may complete
normally when normal end-states (CONCLUDED) are added in future commits.

Notably, Paused/Standby jobs do not transition directly to aborting,
as they must wake up first and cooperate in their cancellation.

Transitions:
Created -> Aborting: can be cancelled (by the system)
Running -> Aborting: can be cancelled or encounter an error
Ready   -> Aborting: can be cancelled or encounter an error

Verbs:
None. The job must finish cleaning itself up and report its final status.

             +---------+
             |UNDEFINED|
             +--+------+
                |
             +--v----+
   +---------+CREATED|
   |         +--+----+
   |            |
   |         +--v----+     +------+
   +---------+RUNNING<----->PAUSED|
   |         +--+----+     +------+
   |            |
   |         +--v--+       +-------+
   +---------+READY<------->STANDBY|
   |         +-----+       +-------+
   |
+--v-----+
|ABORTING|
+--------+

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>