zsh completion doesn't work with sudo

- [x] I have read through the manual page (`man fzf`). But that doesn't apply anyway...
- [x] I have the latest version of fzf
- [x] I have searched through the existing issues

## Info

- OS
- [x] GNU/Linux
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Shell
- [ ] bash
- [x] zsh
- [ ] fish

## Problem / Steps to reproduce

Zsh completion for custom programs doesn't work when `sudo` is the first word on the command line. Usual completion with zsh does support that behaviour on the other hand, i.e. completion for some program name `foo` will work even though `sudo` is the first word on the command line.

However, this is not limited to custom definitions. The case of `kill` doesn't work. For instance, writing

```
$ kill **
```

works, but the following doesn't:

```
$ sudo kill **
```

Start by reproducing completion for `kill **` and `sudo kill **` in zsh on GNU/Linux, then inspect the zsh completion integration for how the command position is detected. Done means custom program completions and the `kill` completion work when `sudo` is the first command-line word.

Written by the indexing model from the issue text.