zsh-completion: add missing -M completion for timedatectl

zsh_completion: Fix single letter args Things like -n to specify the lines to show with systemctl and journalctl accepts syntax like: journalctl -n4 systemctl -n14 Previously, typing `-nXX <tab>` where XX is a number, zsh would try to complete an integer. Now it will see the XX and use the _journalctl_none completion. This is also how any of the single letter options that take arguments work as well.

zsh_completion: Move helper function to autoload _hosts_or_user_at_host was used by 6 different completions, and previously was in all 6 of those files. I moved it out to its own file, _sd_hosts_or_user_at_host. This will be autoloaded for use in other completion functions. It also allows external completions to use this function by simply calling _sd_hosts_or_user_at_host as in the systemd completions.

zsh_completion: Split out zsh _timedatectl