enhancement: systemd socket activation, service and target files

# Context

I wanted to set up service for occasional testing across 5 computers in home network,
xinetd/inetd style, but systemd is the new way of doing things. A bit of reading here http://0pointer.de/blog/projects/socket-activation2.html suggests its not too hard,
but its always easier to ask first.

* Version of iperf3:

3.5. But I searched issues for systemd, which any feature request would refer to.
And this premature submission was accidental... updating incrementally..

* Hardware:

linux on x86 & rpi

* Operating system (and distribution, if any):

Name : iperf3
Version : 3.5
Release : 1.fc28
Architecture: x86_64
Install Date: Thu 24 May 2018 02:51:33 PM MDT

# Enhancement Request

* Current behavior

xinetd operation works iirc (tried many years ago).
I think this means that systemd compatibility is close to trivial.

* Desired behavior

use socket opened by systemd
then various systemd features can help workaround other bugs
(like too many open files) where wait/nowait (inetd vocabulary) can fix.

* Implementation notes

http://0pointer.de/blog/projects/socket-activation.html
http://0pointer.de/blog/projects/socket-activation2.html