[ticket/11049] Ability to force users to fill in the captcha when posting

This removes the use of the enable_post_confirm config setting and replaces
it with a local permission setting (f_nocaptcha).

Works the same as f_noapprove

Database updater handles setting this permission to never for any forums
guests are able to post in.

http://tracker.phpbb.com/browse/PHPBB3-11049