add newsletter checkbox

[Marc-pi]

in the user profile, the user can check/uncheck this option (subscribe/unsubscribe)

the question is about registration : it can scare user to have many input fields in user registration
we cannot skip Privacy/Term&Condition checkbox from registration page, but for newsletter, it's perhaps better to leave out of the registration form.

the better is perhaps to use the Register complete feature of User module
Require user to complete register data in an extra form

actually, when using this feature, we have to hack.