stage1: generate socket unit for socket-activated ports

 * Generate a systemd socket unit for socket-activated ports
 * Add socket-related path generators and cleanup rest
 * Remove unnecessary systemd unit path mkdirs, just trust the
   stage1 rootfs provides them.
hmm, how about "aciID"? (ugh, capitalisation is awkward)
I am not too bothered, happy to ship as-is and if it bothers me later I'll come back to it :P