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
