qrexec-daemon: optionally don't wait for connection to the VM

Will be used to keep qrexec-daemon running in the background even if no
qrexec-agent installed (yet). Specifically to give the VM a chance to
inform about just installed agent.

qrexec/qrexec-daemon.c

@@ -147,6 +147,8 @@ void init(int xid)
 	case 0:
 		break;
 	default:
+		if (getenv("QREXEC_STARTUP_NOWAIT"))
+			exit(0);
 		fprintf(stderr, "Waiting for VM's qrexec agent.");
 		for (i=0;i<startup_timeout;i++) {
 			sleep(1);