We already had nm_free_secret() to clear the secret out
of a NUL terminated string. That works well for secrets
which are strings, it can be used with a cleanup attribute
(nm_auto_free_secret) and as a cleanup function for a
GBytes.

However, it does not work for secrets which are binary.
For those, we must also track the length of the allocated
data and clear it.

Add two new structs NMSecretPtr and NMSecretBuf to help
with that.