networking.service: Add ExecStart=/sbin/ifup -a --allow=hotplug. Patch by...

Santiago R.R.requested to merge
santiago/ifupdown:1022843-systemctl-restart-hotplug into master

networking.service: Add ExecStart=/sbin/ifup -a --allow=hotplug. Patch by "Oleg A. Arkhangelsky" sysoleg@yandex.ru

Closes: #1022843

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