Auxiliary: Bash-comp: use _init_completion instead of _split_longopt

Using `_init_completion -s` will handle the option splitting and set the
`split` var [1].

Keep setting `split` manually for the older manual bash completion
initialization.

[1] https://github.com/scop/bash-completion/blob/main/bash_completion.d/000_bash_completion_compat.bash#L227