LU-7122 utils: changelog_{de}register cleanup

LU-7122 utils: changelog_{de}register cleanup

Document the -n switch for "changelog_register" in the man page.
Apply coding style and remove unneeded code.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I38431d371bc08f5068e5b7e3e62a7847dc64283d
Reviewed-on: http://review.whamcloud.com/16341
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>