Support systemd's socket activation FD passing

[zimbatm]

systemd also has a feature for passing sockets to a program. http://0pointer.de/blog/projects/socket-activation.html

It would be nice to have interoperability on that front.

[roberth]

There are various levels of support for this. fd://3 gets a lot done on the systemd <-> socketmaster end. #18 implements support for 1 socket on the socketmaster <-> child side. Makes for an MVP, but full systemd socket activation support would imply

• multiple fds
• named fds
• easy to use and well documented