Add a way to define the cookie policy.

This adds the -a flag to define a string of the toggle string for the cookie
policy modes. There is now a new »cookiepolicies« string in config.h and the
Mod+Shift+a now can toggle the policy but will not cause a reload, because
this would only add a burden when toggling through accept and not accept.

Thanks Quentin Rameau <quinq.ml@gmail.com> for the suggestions!