llama : add --completion-bash option

[danbev]

This commit adds a new option --completion-bash to the llama.cpp which outputs a source-able bash completion script.

The motivation for this change is to provide a more user-friendly experience for users who use the command-line interface of llama.cpp.

This is currently only basic and all options are displayed for all llama executables but this can be improved in the future if needed.

Example usage:

$ build/bin/llama-cli --completion-bash > ~/.llama-completion.bash
$ source ~/.llama-completion.bash

$ ./build/bin/llama-server --m<TAB>
--main-gpu         --mirostat         --mirostat-lr      --model            --multiline-input
--min-p            --mirostat-ent     --mlock            --model-url