Specifying SIG_IGN for SIGCHLD (which by default, is ignored) is not the same
as specifying SIG_DFL.  See the NOTES section of wait(2) for the skinny on this.

--- Linux-PAM-0.77/modules/pam_pwdb/support.-c	2003-07-09 00:15:06.000000000 -0400
+++ Linux-PAM-0.77/modules/pam_pwdb/support.-c	2003-07-09 00:15:19.000000000 -0400
@@ -370,7 +370,7 @@
 	 * The "noreap" module argument is provided so that the admin can
 	 * override this behavior.
 	 */
-	sighandler = signal(SIGCHLD, SIG_IGN);
+	sighandler = signal(SIGCHLD, SIG_DFL);
     }
 
     /* fork */
--- Linux-PAM-0.77/modules/pam_unix/support.c	2003-07-09 00:15:29.000000000 -0400
+++ Linux-PAM-0.77/modules/pam_unix/support.c	2003-07-09 00:15:41.000000000 -0400
@@ -597,7 +597,7 @@
 	 * The "noreap" module argument is provided so that the admin can
 	 * override this behavior.
 	 */
-	sighandler = signal(SIGCHLD, SIG_IGN);
+	sighandler = signal(SIGCHLD, SIG_DFL);
     }
 
     /* fork */