- Changed return value in userspace stream set_option to "not implemented",

- Changed return value in userspace stream set_option to "not implemented",
  instead of failure.
#Currently, there's no way to test this, because the only operations that have
#a default implementation, set_chunk_size and set_read_buffer are either not
#exposed or, in the 2nd case, completely delegated to the user implementation,
#which can only return true/false, not "not implemented" (and not implementing
#the set_option method in the userspace stream results in error).