logind: flip KillUserProcesses to on by default

logind: flip KillUserProcesses to on by default

This ensures that users sessions are properly cleaned up after.
The admin can still enable or disable linger for specific users to allow
them to run processes after they log out. Doing that through the user
session is much cleaner and provides better control.

dbus daemon can now be run in the user session (with --enable-user-session,
added in 1.10.2), and most distributions opted to pick this configuration.
In the normal case it makes a lot of sense to kill remaining processes.
The exception is stuff like screen and tmux. But it's easy enough to
work around, a simple example was added to the man page in previous
commit. In the long run those services should integrate with the systemd
users session on their own.

https://bugs.freedesktop.org/show_bug.cgi?id=94508
https://github.com/systemd/systemd/issues/2900