Support applying zstyle customizations for perforce_labels and perforce_clients similar to perforce_changes

[kting28]

Hello, In my use case our perforce server has a lot of clients and labels so calling p4 clients or p4 labels with no arguments is guaranteed to hang the process. The documentation at the top of _perforce mentioned support of zstyle customizations for perforce_changes (e.g. limiting to a particular user). This pull request is just to extend the support to p4 labels and p4 clients so users can limit the range of these instructions through zstyle customizations.

[danielshahaf]

Just noticed that _perforce lists pws as maintainer (line 4). He doesn't seem to have a github account,¹ but we can get in touch with him via -workers@. So, @kting28, if a docs patch is coming up, would it be possible for you to post it to -workers@, so pws sees it? I realize we document that completion patches are welcome on github, but someone who is listed as maintainer deserves a heads up.

¹ On https://github.com/zsh-users/zsh/commits/master his name isn't a hyperlink.