Socket-activate our services

Socket activation of our services bring the following benefits:

• Security: No additional privileges or capabilities needed for sockets (quite nice for the executor)
• Reliability: By having the sockets managed by systemd, services can crash or be restarted without having to stop the services that depend on it... any new connection will simply be blocked until the restart has happened.

The latter point is especially important, as it is difficult to tell systemd that the executor needs to be stopped whenever salad gets rebooted or failed for any reason... then have the executor start again when salad comes back to life. By moving the dependency requirement to salad.socket, we can ensure that this will never crash and salad.service will be restarted without further affecting the executor.

References: #27 (closed)