LU-12734 misc: allow older bash_completion versions

LU-12734 misc: allow older bash_completion versions

Allow the "lctl" bash_completion to work on older versions which
which don't have _init_completions().  Check at runtime if this
function is available, and if not fall back to an older interface.

Has been manually tested with both bash-completion v1.3 and v2.1.

Fixes: f87a7f2656ce ("LU-12734 misc: add bash completion for lctl set/get_param)"
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3822c0967354d83d12f299c4be3023b2fc254035
Reviewed-on: https://review.whamcloud.com/36459
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Dominique Martinet <dominique.martinet@cea.fr>