do rfork(RFNOTEG) before exec

src/libthread/exec.c

@@ -75,6 +75,7 @@ _threadspawn(int fd[3], char *cmd, char *argv[])
 		close(p[1]);
 		return -1;
 	case 0:
+		rfork(RFNOTEG);
 		dup2(fd[0], 0);
 		dup2(fd[1], 1);
 		dup2(fd[2], 2);