ps/display.c: Always exit from signal_handler().

Right now, "we _exit() anyway" is not always true: for example, the
default action for SIGURG is to ignore the signal, which means that
"kill(getpid(), signo);" does not terminate the process. Call _exit()
explicitly, in this case (rather than exit(), because the terminating
kill() calls do not call the functions registered with atexit() either).

ps/display.c

@@ -66,6 +66,7 @@ static void signal_handler(int signo){
       error_at_line(0, 0, __FILE__, __LINE__, "%s", _("please report this bug"));
       signal(signo, SIG_DFL);  /* allow core file creation */
       kill(getpid(), signo);
+      _exit(EXIT_FAILURE);
   }
 }