Don't trigger udev if socket doesn't exist
    
    On rpm-ostree systems, we don't want scriptlets to affect the running
    system because a major part of the value is "background updates".
    Scriptlets are run in a containerized environment where e.g. udev is not
    available.
    
    Add a check for the udev socket before triggering it to handle this.
    This also helps the container use case.
    
    See also: https://bugzilla.redhat.com/show_bug.cgi?id=1352154
    See also: https://src.fedoraproject.org/rpms/udisks2/pull-request/3
    See also: https://github.com/coreos/fedora-coreos-tracker/issues/703