bash-completion: split completions and move to new location

Split the large bash completion script into separate, smaller files each
named after the binary it is used for and move the files to
/usr/share/bash-completion/completions. This way the completions can be
loaded on demand and we only install the completions for the tools we
actually build. The old path /etc/bash_completion.d/ is deprecated and
will disappear in the future.
Update TODO