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/base/thread/thread_linux.cc

@@ -40,6 +40,11 @@ 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();
 }

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();
 }

base-linux/src/platform/linux_syscalls.h

@@ -314,6 +314,7 @@ inline int lx_stat(const char *path, struct stat64 *buf)
 
 enum {
 	LX_SIGUSR1   = 10,  /* used for cancel-blocking mechanism */
+	LX_SIGCHLD   = 17,  /* child process changed state, i.e., terminated */
 	LX_SIGCANCEL = 32,  /* accoring to glibc, this equals SIGRTMIN,
 	                       used for killing threads */
 };

base-linux/src/platform/lx_hybrid.cc

@@ -167,6 +167,11 @@ static void adopt_thread(Thread_meta_data *meta_data)
 	 */
 	lx_sigaction(LX_SIGUSR1, empty_signal_handler);
 
+	/*
+	 * Prevent children from becoming zombies. (SIG_IGN = 1)
+	 */
+	lx_sigaction(LX_SIGCHLD, (void (*)(int))1);
+
 	/* assign 'Thread_meta_data' pointer to TLS entry */
 	pthread_setspecific(tls_key(), meta_data);