Add _Z_NO_COMPLETE_CD option

I don't like getting 'z' completions for 'cd', because I often use cd[space][tab] to cd to the first subdirectory, so I added an option to disable that. Define _Z_NO_COMPLETE_CD to disable 'cd' completion, leave undefined for old behavior.
Fix zsh compatibility

Oops.
Add a tunable _Z_NO_COMPLETE_CD to disable advanced completion for cd.

Proposed by @K900 on Github.
Merge remote-tracking branch 'K900/patch-1'