Socket Activation Support

[DocterDum]

I'm running OFSM as a systemd unit and would like to use systemd's socket activation feature to manage starting the server.

When the current binary is run in this context it crashes because it cannot bind to the port (Systemd binds the port and expects to hand the socket to OFSM, but OFSM tries to create a new socket, fails, and crashes).

An example solution can be seen on this page: https://vincent.bernat.ch/en/blog/2018-systemd-golang-socket-activation