manager: Try looking up session based on PID first

Unfortunately, GDM may be running multiple greeters, and each greeter is
currently using the same user. So while in a lot of setups each user
should only have one graphical session and also only one DBus session
bus, this is not true for the gdm greeter.

Lacking another solution (e.g. separate users), we need to be able to
correctly lookup the session information for all greeter instances. We
can do so by using sd_pid_get_session and using this information is safe
if it does return something.

See: #526

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