rsync timeout handling

[ewxrjk]

rsync-timeout kills the rsync process after a time interval, imposing an upper bound on how long a backup can take. That's not useless but it's often not what you want.

There should, additionally, be an idiomatic way to access rsync's built-in --timeout option.

rsync-timeout would be the natural name for it but that's taken. The new behaviour will be:

• rsync-io-timeout means the rsync --timeout option
• backup-job-timeout means the behaviour now called --rsync-timeout
• rsync-timeout will be deprecated (but keep its current behaviour at least for the usual 2Y deprecation window)