Now that the secret-key is hashed with the machine-id, the name is
no longer best.

Sure, part of the key are persisted in /var/lib/NetworkManager/secret_key
file, which the user is well advised to keep secret.

But what nm_utils_secret_key_get() returns is first and foremost a binary
key that is per-host and used for hashing a per-host component. It's
really the "host-id". Compare that to what we also have, the
"machine-id" and the "boot-id".

Rename.