Add systemd socket activation capability

Add support for a systemd.socket activation. Socket it passed from
systemd via file descriptor (see systemd.socket doc)
Also, added a few features for convenience:
  * --conn/conv-timeout options to control conversation and connection
    timeouts
  * --shutdown option to shutdown service after all connections are
    timeouted. This is for convent usage with systemd.socket
  * test script which allows to smoke test udp speeder and its
    socket activation capability
  * example systemd unit files

examples/udp_speeder.service

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+[Unit]
+Description=UDP Speeder Service
+After=network.target
+
+[Service]
+ExecStart=/home/mkarpets/repos/UDPspeeder/speederv2 -s -l0.0.0.0:4096 -r127.0.0.1:7777 -f20:10 --log-level 4 --conv-timeout 10 --conn-timeout 20 --shutdown
+
+[Install]
+WantedBy=multi-user.target

examples/udp_speeder.socket

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+[Unit]
+Description=UDP Speeder Socket
+
+[Socket]
+ListenDatagram=0.0.0.0:4096
+FreeBind=true
+
+[Install]
+WantedBy=sockets.target