Allow skipping of security checks for compinit in ZSH initialisation

This release includes the following:

• Issue #964: With ZSH_DISABLE_COMPFIX="true", compinit -u -C is called. The -C flag will cause compinit to skip all security checks and the -u flag will use all files found for completion. This is considered best practice in oh-my-zsh.
• Some pre-emptive work to enable checksum cabability.