[RFC] [WIP] runit: respect SVDIR env variable in completion

[crater2150]

Allows sv's completion to complete other service directories, e.g. for per-user services.
If the $SVDIR environment variable is set and sv isn't used with sudo, the completion is changed to complete services from $SVDIR. Otherwise behaviour is unchanged.

I currently have only implemented the change for zsh. In bash, the sudo completion modifies variables so that the completion of the following command doesn't "see" the sudo in the beginning. If there is interest in merging this, I'd check for a bash solution too.

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR

[skip CI]
Does not change any compiled files

[ericonr]

Please open this in https://github.com/void-linux/runit instead.

CI is cheap for runit, there's really no reason to skip it :)

I think the change makes sense, so please do reopen this in that repo :)