Add a simple signal handler for SIGCHLD on Unix

This fixes #6631

(cherry picked from commit cff6840ff7da010112b94f9be13deaa8288e90cd)

drivers/unix/os_unix.cpp

@@ -115,6 +115,13 @@ int OS_Unix::unix_initialize_audio(int p_audio_driver) {
 static MemoryPoolStaticMalloc *mempool_static=NULL;
 static MemoryPoolDynamicStatic *mempool_dynamic=NULL;
 	
+// Very simple signal handler to reap processes where ::execute was called with
+// !p_blocking
+void handle_sigchld(int sig) {
+	int saved_errno = errno;
+	while (waitpid((pid_t)(-1), 0, WNOHANG) > 0) {}
+	errno = saved_errno;
+}
 	
 void OS_Unix::initialize_core() {
 
@@ -146,6 +153,14 @@ void OS_Unix::initialize_core() {
 
 	ticks_start=0;
 	ticks_start=get_ticks_usec();
+
+	struct sigaction sa;
+	sa.sa_handler = &handle_sigchld;
+	sigemptyset(&sa.sa_mask);
+	sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
+	if (sigaction(SIGCHLD, &sa, 0) == -1) {
+		perror("ERROR sigaction() failed:");
+	}
 }
 
 void OS_Unix::finalize_core() {