LU-5504 utils: add const qualifier to changelog accessors.

LU-5504 utils: add const qualifier to changelog accessors.

Commit 6e1365 changed 4 functions, and commit 0f22e4 accidentally
reverted them.

This patch put them back, as well as adding new ones to
changelog_rec_size(), changelog_rec_varsize(), changelog_rec_rename()
and changelog_rec_jobid().

6e1365 also changed changelog_rec_name() and changelog_rec_sname() to
return a const, but that is not possible anymore.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: I19e389c422795c2ece4d7af369099cb733d3cb1a
Reviewed-on: http://review.whamcloud.com/13787
Tested-by: Jenkins
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>