Remove dependency on systemd

[rail5]

Currently, the autobuild daemon runs via a systemd "socket."

The socket lives as a file in /var/run. When a process writes to the socket, systemd launches the autobuild daemon and routes stdin/stdout through the socket

This works great, except on any system which doesn't use systemd. Devuan, AntiX, etc

[rail5]

Incidentally, providing our own proper daemon (rather than using socket-based activation) would help us handle queuing better, without risking running too many processes at once. See #53

This daemon could still be configured to be handled by systemd on systems which use systemd, but could also have proper SysVInit scripts etc for use elsewhere

We wouldn't have to move outside of shell scripting in order to construct such a daemon. It should be possible to have a daemon which creates & listens to a socket file by using socat, which we already use for the autobuild client.