Disabled by default (configurable by user preference).

Also, reorganise erp5_auto_logout code:
- logout: Do not duplicate original code.
- twiddleAuthCookie: Simplify & factorise.
- setAuthCookie: Resync features with Cookie Crumbler's, and set http_only flag.
- ERP5Site_getMaxUserInactivityDuration: Cached access takes the same time as actual access (and access is surprisingly slow, actually), so do not cache.
- Base_getAutoLogoutSessionKey: New script, to factorise session key generation.