Fix annoying tab-completion in Zsh with dotglob

When `setopt globdots` is enabled in Zsh with Oh-My-Zsh, the "special
directories" ("." and "..") are included in the options by default.
This commit disables that behaviour.

The special-dirs option is rarely, if ever, going to be useful as if
either "." or ".." is ever needed it's less trouble to type them
manually; meanwhile their inclusion is a persistent annoyance,
especially in directories with only one entry where tab completion
would otherwise be streamlined.