core,logind,networkd: don't pick up devices from udev before they finished udev initialization

Managers shouldn't pick up the devices the manage before udev finished
initialization, hence check explicitly for that.
comment out udev's is_initialized call until the problem is sorted out
login: Don't stop a running user manager from garbage-collecting the user.

With the current logic, a user will never be garbage-collected, since its
manager will always be around. Change the logic such that a user is
garbage-collected when it has no sessions and linger is disabled.