Linux: prevent zombie state for child processes

Setting the handler for SIGCHLD to SIG_IGN (ignore) informs the kernel
not to enter the zombie state: (man 2 wait)

  POSIX.1-2001 specifies that if the disposition of SIGCHLD is set to
  SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see
  sigaction(2)), then children that terminate do not become zombies
  [...]

Fixes #271.

base-linux/src/core/thread_linux.cc

@@ -32,6 +32,11 @@ static void thread_start(void *)
 	 */
 	lx_sigaction(LX_SIGUSR1, empty_signal_handler);
 
+	/*
+	 * Prevent children from becoming zombies. (SIG_IGN = 1)
+	 */
+	lx_sigaction(LX_SIGCHLD, (void (*)(int))1);
+
 	Thread_base::myself()->entry();
 	sleep_forever();
 }