session: ignore pam info messages for automatic sessions
    
    launch-environment sessions and autologin sessions don't have
    a greeter associated with them to ask the user questions, or
    to display PAM messages. It's assumed the PAM stack will be
    set up in such a way that no conversing will be required.
    
    This assumption isn't always correct, if the system pam configuration
    includes something like pam_lastlog.
    
    This commit makes us discard info messages silently rather than
    fail.
    (cherry picked from commit 51b03d450875e82db29f905ac7e0fa12bed79304)