Clean up transmission-remote-cli-bash-completion.sh

I am planning to move the bash completion script from `/etc/bash_completion.d/transmission-remote-cli-bash-completion.sh` to `/usr/share/bash-completion/completions/transmission-remote-cli` in the next Debian package upload.

I've fixed up the bash completion script so that it conforms with the rest of the bash completion scripts in the new directory, and moved it to its own directory in the repository so that other completion scripts can be added for other shells such as ZSH without cluttering the top level of the repository.

Jon