systemd socket activation

[dun]

Add support for systemd socket activation. This would allow munged to be restarted without potentially dropping any incoming requests during that time. It would also allow for the initial phase of Key Rotation to rotate to a new key by restarting the daemon.

• http://0pointer.de/blog/projects/socket-activation.html
• http://0pointer.de/blog/projects/socket-activation2.html
• http://0pointer.de/blog/projects/socket-activated-containers.html
• http://0pointer.de/blog/projects/inetd.html
• http://0pointer.de./public/systemd-man/daemon.html#Socket-Based%20Activation
• https://fedoraproject.org/wiki/Packaging:Systemd#Socket_activation
• https://freedesktop.org/wiki/Software/systemd/DaemonSocketActivation/