logind: introduce "user-light" session class

logind: introduce "user-light" session class

This new session class is to "user" what "background" is to
"background-light": it doesn't cause the per-user service manager to
start.

This new session class is now the default if no session class was
provided at session registration time and the following conditions hold:

1. The session is not graphical
2. The user is not a regular user (but not root)

Or in other words root and system users won't get a service manager
started automatically if they go through a PAM session as part of things
like cron or ftp. They will however still get one if they log in
graphically.

This changes behaviour a bit, but hopefully in OK was.

This also makes "background-light" for system users incl. root.

This addresses one of the ideas discussed in #34988.