Listen to dying child

An r3940 vtest failure on freebsd seems to suggest that the child
does not terminate while blocking in a write.

***  v1    debug|Error: Child (74042) Panic at: Mon, 03 Jul 2023 13:40:40 GMT
***  v1    debug|Assert error in CLI_Run(), cache/cache_cli.c line 102:
***  v1    debug|  Condition((VCLI_WriteResult(heritage.cli_out, CLIS_OK, \"Ready\")) == 0) not true.

This might need another iteration.

bin/varnishd/mgt/mgt_child.c

@@ -558,8 +558,10 @@ mgt_reap_child(void)
 	vsb = VSB_new_auto();
 	XXXAN(vsb);
 
+	(void)VFIL_nonblocking(child_output);
 	/* Wait for child to die */
 	for (i = 0; i < mgt_param.cli_timeout * 10; i++) {
+		(void)child_listener(NULL, VEV__RD);
 		r = waitpid(child_pid, &status, WNOHANG);
 		if (r == child_pid)
 			break;