[kernel] Only accept invalid handles for optional syscall args

The syscall helpers.h get_handle functions should be returing
j6_err_invalid_arg if the handle they're given is j6_handle_invalid,
unless explicitly set to optional.

src/user/drv.uart/main.cpp

@@ -41,7 +41,7 @@ int
 channel_pump_loop()
 {
     j6_status_t result;
-    constexpr size_t buffer_size = 512;
+    static constexpr size_t buffer_size = 512;
     char buffer[buffer_size];
 
     j6_handle_t slp = j6_find_first_handle(j6_object_type_mailbox);