systemd support

First attempt at systemd support; organized in two service (init) files, and six config files. This is because I haven't found a way to run two daemons from one service file (without it halting indefinitely - one process then waits for the other to finish). 

The backup is started by the main service, as "Wants"; this means if the backup fails to start the main won't also fail. Variables are based on the official Arch Linux dnscrypt-proxy package. So far, I've tested config-dnscrypteu in Arch.