util/bash-completion: Fix for bash-completion 2.12

util/bash-completion: Fix for bash-completion 2.12

_split_longopt() was the bash-completion private API and removed since
bash-completion 2.12. This commit initializes the bash-completion
general variables with _init_completion() to avoid the potential
"command not found" error.

Although bash-completion 2.12 introduces _comp_initialize() to deprecate
_init_completion(), _init_completion() is still chosen for the better
backward compatibility.

Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>