Cookie expire time

As discuted in #118 about enforcing cookie expire time, here an implementation.

- add cookie-expire-time configuration setting
- document the new setting
- with cookie-expire-time=0 : cookies are converted to session-cookies (expires=NULL)
- with cookie-expire-time>0 : cookies' expire-time is the lower between server-side request and cookie-expire-time
- with cookie-expire-time<0 : the expire time is left as server-side define it. It is the default.