Fix logind startup

Fix logind startup

* src/login/logind.c (manager_connect_bus):
  - Notice instead of error if we can't subscribe to updates from
    systemd. Perhaps we should remove this entirely. But leaving
    it optional means, that a system managed by systemd can use
    elogind substituting systemd-login.
  - Warn instead of error if we can't add receiver matches from
    systemd events. On a system not run by systemd, such events
    wouldn't occur anyway.