Use main() return value as exit code

include/vm.h

@@ -97,7 +97,7 @@ int vm_string_ref(struct string *s);
 int vm_copy_page(struct page *page);
 union vm_value vm_copy(union vm_value v, enum ain_data_type type);
 
-void vm_execute_ain(struct ain *program);
+int vm_execute_ain(struct ain *program);
 void vm_call(int fno, int struct_page);
 int vm_time(void);
 

src/system4.c

@@ -352,6 +352,5 @@ int main(int argc, char *argv[])
 	dbg_init();
 #endif
 
-	vm_execute_ain(ain);
-	sys_exit(0);
+	sys_exit(vm_execute_ain(ain));
 }

src/vm.c

@@ -1581,7 +1581,7 @@ static void vm_execute(void)
 	}
 }
 
-void vm_execute_ain(struct ain *program)
+int vm_execute_ain(struct ain *program)
 {
 	// initialize VM state
 	stack_size = INITIAL_STACK_SIZE;
@@ -1631,6 +1631,7 @@ void vm_execute_ain(struct ain *program)
 	}
 
 	vm_call(ain->main, -1);
+	return stack_pop().i;
 }
 
 void vm_stack_trace(void)