LU-7670 mdt: allow changelog commands to return errors

Return errors to lctl/lfs users for out of range, and invalid IDs.
Currently only 0 is ever returned, regardless of outcome.  Some
information is printed in the MDS logs, but nothing on the client
to indicate success or failure.

Split purge and clear code into 2 separate paths.

Signed-off-by: Ben Evans <bevans@cray.com>
Change-Id: Ie139b170da0c8bef1c315ddb5361783230bb51ad
Reviewed-on: https://review.whamcloud.com/18030
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>