set O_NONBLOCK flag to stdout

dwl.c

@@ -2190,6 +2190,12 @@ run(char *startup_cmd)
 		close(piperw[1]);
 		close(piperw[0]);
 	}
+
+	/* Mark stdout as non-blocking to avoid people who does not close stdin
+	 * nor consumes it in their startup script getting dwl frozen */
+	if (fd_set_nonblock(STDOUT_FILENO) < 0)
+		close(STDOUT_FILENO);
+
 	printstatus();
 
 	/* At this point the outputs are initialized, choose initial selmon based on