login: revert lazy session-activation on non-VT seats

Existing applications like gdm already depend on new sessions to get
immediately activated on seats without VTs. Fixes a bug reported as:
  [systemd-devel] systemd 208:trouble with inactive user sessions at non-seat0 seats

This patch restores the original behavior. We either need to add a new
flag for session-creation or some other heuristic to avoid activating new
sessions in the future.