sbus/interface: fixed interface copy helpers
    
    In `sbus_method_copy()` and other copy helpers there was code like:
    ```
    copy = talloc_zero_array(mem_ctx, struct sbus_method, count + 1);
    memcpy(copy, input, sizeof(struct sbus_method) * count + 1);
    ```
    Copy of one byte of "sentinel" doesn't make a sense.
    We can either rely on the fact that sentinel is zero-initialized struct
    *and* `talloc_zero_array()` zero-initializes memory (so copying of
    sentinel may be omitted at all) or just copy sentinel in a whole.
    Opted for second option as more clear variant.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>