Add compile option to disable internal handling of fatal signals

By default, charon and its derivatives internally handle the SIGSEGV,
SIGILL, and SIGBUS signals raised by threads (segv_handler).  Add a compile
option so that the signal handling can optionally be done externally.

Closes strongswan/strongswan#132.

src/charon-cmd/charon-cmd.c

@@ -174,6 +174,7 @@ static bool lookup_uid_gid()
 	return TRUE;
 }
 
+#ifndef DISABLE_SIGNAL_HANDLER
 /**
  * Handle SIGSEGV/SIGILL signals raised by threads
  */
@@ -189,6 +190,7 @@ static void segv_handler(int signal)
 	DBG1(DBG_DMN, "killing ourself, received critical signal");
 	abort();
 }
+#endif /* DISABLE_SIGNAL_HANDLER */
 
 /**
  * Print command line usage and exit
@@ -372,18 +374,23 @@ int main(int argc, char *argv[])
 	/* handle all arguments */
 	handle_arguments(argc, argv, FALSE);
 
-	/* add handler for SEGV and ILL,
-	 * INT, TERM and HUP are handled by sigwaitinfo() in run() */
-	action.sa_handler = segv_handler;
+	/* add handler for fatal signals,
+	 * INT, TERM, HUP and USR1 are handled by sigwaitinfo() in run() */
 	action.sa_flags = 0;
 	sigemptyset(&action.sa_mask);
 	sigaddset(&action.sa_mask, SIGINT);
 	sigaddset(&action.sa_mask, SIGTERM);
 	sigaddset(&action.sa_mask, SIGHUP);
 	sigaddset(&action.sa_mask, SIGUSR1);
+
+	/* optionally let the external system handle fatal signals */
+#ifndef DISABLE_SIGNAL_HANDLER
+	action.sa_handler = segv_handler;
 	sigaction(SIGSEGV, &action, NULL);
 	sigaction(SIGILL, &action, NULL);
 	sigaction(SIGBUS, &action, NULL);
+#endif /* DISABLE_SIGNAL_HANDLER */
+
 	action.sa_handler = SIG_IGN;
 	sigaction(SIGPIPE, &action, NULL);