zsh_completion: Allow specifying multiple arguments

zsh_completion: Allow specifying multiple arguments

Some of the options in systemd can take multiple arguments, such as
systemctl's --type option. Previously, you would only be able to
complete a single type after the -t, but now zsh will continue to
complete the types, separating them by commas.

systemd-inhibit's --what command has colon (:), and that has been taken
into account.