and add nm_utils_secret_key_get() to cache the secret-key, to only
obtain it once.

nm_utils_secret_key_read() is not expected to fail. However, in case
of an unexpected error, don't propagate the error to the caller,
but instead handle it internally.

That means, in case of error:
  - log a warning within nm_utils_secret_key_read() itself.
  - always return a generated secret-key. In case of error, the
    key won't be persisted (obviously). But the caller can ignore
    the error and just proceed with an in-memory key.

Hence, also add nm_utils_secret_key_get() to cache the key. This way,
we only try to actually generate/read the secret-key once.  Since that
might fail and return an in-memory key, we must for future invocations
return the same key, without generating a new one.