kymnob

- [ ] Perhaps a few more adjustments will be needed once void-linux/xmirror#16 and #590 are resolved. - [ ] In the end, I'd like to exclude long-form options (`--debug`)...

While I was updating `zsh` completion, I noticed a few inconsistencies between `--help` and man pages: | `xbps-dgraph` | `--help` | `man` | ----------------- | -------- | --------- | `MODE`...

This PR adds optional repository `zsh` completion to `_xmirrors` to re-use in `_xbps`.