Make Security Questions included on login form optional

I updated the code in security_questions.module for D6 to make it optional whether or not the security questions should be included on the login screen. My testing indicates it works. I basically took what chertzog did to make the Password Reset sequence optional.

However, I did update the install script as well to make it Enabled by default. My patch attached to this issue also includes the changes implemented in #1617174: Add additional default questions to the installation.

Take it or leave it, but in my (client's) use case, we probably don't want the security questions on the login page, but only be used on the password reset form.