Fix #299: add support for service readiness notification

This patch adds service readiness notification to support daemons
employing systemd and s6 notification.  Complementing the native
Finit readiness support using PID files that exist already.

The two have slightly different ways of implementing readiness:

 - https://www.freedesktop.org/software/systemd/man/sd_notify.html
 - https://skarnet.org/software/s6/notifywhenup.html

Finit now provides both a NOTIFY_SOCKET environemnt variable, for
systemd, and a way to start s6 daemons with a descriptor argument.

For details on the syntax, see the `service` documentation.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/api.c

@@ -509,6 +509,20 @@ static void api_cb(uev_t *w, void *arg, int events)
 			result = do_signal(rq.data, sizeof(rq.data), rq.runlevel);
 			break;
 
+		case INIT_CMD_NOTIFY_SOCKET:
+			svc = svc_find_by_pid(rq.runlevel);
+			if (!svc) {
+				errx(1, "Unknown PID, cannot register notify socket");
+				result = 1;
+				break;
+			}
+
+			if (uev_io_init(ctx, &svc->notify_watcher, service_notify_cb, svc, sd, UEV_READ)) {
+				err(1, "Falied initializing %s readiness notifier", svc_ident(svc, NULL, 0));
+				break;
+			}
+			return;	/* Don't close sd, used for notify */
+
 		default:
 			dbg("Unsupported cmd: %d", rq.cmd);
 			break;