SystemD socket activation

[tW4r]

I had an idea about how lazymc could use even less resources by using SystemD socket activation:

The service would be off by default consuming no resources
When a request comes in on a given port (25565) SystemD would automatically start lazymc and pass it the socket
If the startup time of lazymc itself (not minecraft server) is low enough this would be transparent to the user
Then lazymc could start minecraft and work normally
After all players logout and some time passes lazymc would stop itself and consume no resources once again

what I think is needed for this to work:
Support a passed open socket by SystemD
Support automatically stopping lazymc after having stopped the minecraft server