pam_systemd: reword message about not creating a session

The message is changed from
  Cannot create session: Already running in a session...
to
  Not creating session: Already running in a session...

This is more neutral and avoids suggesting a problem.

"Will not create session: ..." was suggested, but it sounds like the action
would have yet to be performed. I think Using present continuous is better.

Fixes #10822 (for good now, I hope).