Independent bash-completion for each executable

[bberberov]

Since b9031, it's now possible to generate bash-completions very quickly. --completion-bash already generates a different set of command line options for each executable. But it assigns the _llama_completions() function to all executables. I think each executable should generate its own completion function/script. Those can be aggregated into a single completion script, or remain separate scripts; packager's choice. I already started working on a patch, but I wanted to check if there is a reason to use a single completion function for all executables. Would you be interested in changing this?