LU-3999 hsm: Improve mdt.*.hsm.agents output in /proc

LU-3999 hsm: Improve mdt.*.hsm.agents output in /proc

mdt.*.hsm.agents file contains a list of registered agents and a
couple of information for each of them. This patch improves the output
to clarify the meaning of each returned values.

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Change-Id: Id4858f7ac9387d539eba03389871c6e99476a5c9
Reviewed-on: http://review.whamcloud.com/7744
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>