Fix 1891: Add zsh command completion generator

Fix 1891: Add zsh command completion generator

This adds the function needed to generate the zsh autocompletion scripts.
Using the bash completion code path, this generates the list of possible
completions and adds them to the zsh completion by compadd.
For the autocompletion scripts to be loaded automatically, the following two
lines have to be placed in the .zshrc:

autoload -U compinit
compinit

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>