The signal "notify:g-name-owner" is only emitted for well-known
names, but not for unique connection names (":1.x") such as the secret
agent's connection. Also, it will not be emited for private connections.

That meant that when the client disconnected without explicitly
unregistering, the NMSecretAgent was not cleaned up and leaked
indefinitely.
As only one instance of secret agent with a certain 'identifier/uid'
pair can register, this bug also prevented the client from registering
until restart of NetworkManager.

Fixes: df670681