pam-systemd: introduce "user-light" session type, and make "background-light" the...

This implements one idea from #34988: default to "user-light" and
"background-light" for system users, so that the service manager is only
pulled in for sessions that likely need them, i.e. not cron jobs or ftp
logins.

This is a compat break to some degree, but I think a worthy one. I
updated the NEWS file to explain this.