runner: Expose a control socket for controlling execution
A feature required for drm/intel#435
igt_runner needs to expose an AF_LOCAL socket when configured to do so and daemonize itself (itself, or with screen or tmux). CI executor will then no longer be required to maintain an ssh connection active for the duration of the whole test round. Instead CI executor will reconnect the ssh connection when/if it fails and check through igt_runner's control channel if a test is still running.
Same control channel can be used to give an abort command to gracefully stop execution between IGT tests.