detect crashes from constraint sub-processes, instead of ignoring them.

if this happens, we want to tear down all of ntpd, so that people will
report it, any such bug can be found, and fixed.
ok bcook

src/usr.sbin/ntpd/constraint.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: constraint.c,v 1.11 2015/05/21 14:24:43 reyk Exp $	*/
+/*	$OpenBSD: constraint.c,v 1.12 2015/05/28 21:34:36 deraadt Exp $	*/
 
 /*
  * Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
@@ -250,7 +250,7 @@ constraint_check_child(void)
 {
 	struct constraint	*cstr;
 	int			 status;
-	int			 fail;
+	int			 fail, sig;
 	pid_t			 pid;
 
 	do {
@@ -258,9 +258,9 @@ constraint_check_child(void)
 		if (pid <= 0)
 			continue;
 
-		fail = 0;
+		fail = sig = 0;
 		if (WIFSIGNALED(status)) {
-			fail = 1;
+			sig = WTERMSIG(status);
 		} else if (WIFEXITED(status)) {
 			if (WEXITSTATUS(status) != 0)
 				fail = 1;
@@ -268,6 +268,10 @@ constraint_check_child(void)
 			fatalx("unexpected cause of SIGCHLD");
 
 		if ((cstr = constraint_bypid(pid)) != NULL) {
+			if (sig)
+				fatalx("constraint %s, signal %d", 
+				    log_sockaddr((struct sockaddr *)
+				    &cstr->addr->ss), sig);
 			if (fail) {
 				log_debug("no constraint reply from %s"
 				    " received in time, next query %ds",