Restore default SIGCHLD handler for spawned processes

Examples of software expecting SIGCHLD not to be ignored at startup are:
- /sbin/ifup & /sbin/ifdown (Debian)
- 'subprocess' module of Python

trigger.c

@@ -6,6 +6,7 @@
 
 #include <string.h>
 #include <ctype.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -221,6 +222,10 @@ void run_triggers(int type, int code, int value, keystate_holder ksh, device *de
 					char ev[8];
 					sprintf( &(ev[0]), "%d", et->value );
 					setenv( "TH_VALUE", &(ev[0]), 1 );
+					/* default SIGCHLD is not automatically
+					   restored and most applications would
+					   fail if this signal is ignored */
+					signal(SIGCHLD, SIG_DFL);
 					system(et->action);
 					exit(0);
 				} else if (pid < 0) {