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.