Change rules config from strings to booleans

Actually I'm beginning to wonder if this is a bad idea. If existing sites are using the config checks like this will break:

'checkMxRecords' => $rules[self::MX_LOOKUP] !== '0',

The breakage will be silent until someone updates a user account.

I think I'll postpone this until we have reason to do a major release.

Major version update is a good time to make this change, reopened.

Automatically closed - issue fixed for 2 weeks with no activity.