Add a new make target (completion) to generate the shell
completion scripts. This will generate the scripts for bash,
zsh and fish for both podman and podman-remote with `podman completion`.
The scripts are put into the completions directory and can be
installed system wide with `sudo make install.completions`.
This commit replaces the current handwritten scripts for bash and zsh.
The `validate.completion` target has been adjusted to make sure nobody
edits these scripts directly.
Signed-off-by: Paul Holzinger <paul.holzinger@web.de>
