zshrc: Put _grmlinitd completion directly into zshrc.

We provide the Start/Stop/... functions in our zshrc. But people don't
necessarily install /usr/share/grml/zsh/completion/grml/_grmlinitd and
therefore the completion is not available then. So lets make sure we
provide the completion through the zshrc file itself.

Reported-by: Othmar Gsenger <otti@anytun.org>