feature-request... add zsh completions for sysctl per man page for sysctl -N

[porteusconf]

The man sysctl for argument -N says:

 -N      Show only variable names, not their values.  This is particularly
         useful with shells that offer programmable completion.  To enable
         completion of variable names in zsh(1) (ports/shells/zsh), use the
         following code:

                   listsysctls () { set -A reply $(sysctl -AN ${1%.*}) }
                   compctl -K listsysctls sysctl

I am trying to figure out how to do this and will update here if if a PR does not beat me to it. (I am more a basher, new to zsh and oh-my-zsh) ;-)