Tidy up the end-of-data handling in backup

* Increase transfer buffer size to 32K
* Add logging about error conditions and fd teardown
* Pass the fd number as a command line option to the 'bu' subprocess
* Properly harvest the 'bu' subprocess after it's done

Change-Id: Id44dde25778ecf43c5604fd9d01d726ba58861e5
3 files changed