openvpn3-indicator should verify session status when user credentials has been collected

If starting a VPN session from the command line (like openvpn3 session-start) which requires user credentials, the indicator will pop up the query box. This is actually quite cool! But if the credentials are provided via the command line, clicking "Cancel" will kill the VPN session.

The openvpn3-indicator should check that the session minor status is still CFG_REQUIRE_USER before doing the disconnect. This pop-up should ideally also detect if the session state changes while being open and close the pop-up automatically if the session changes to any of the other CONN_* minor statuses.

The openvpn3 command line will also need to be adopted a bit to also avoid similar misbehaviour; I'll tackle that as part of the GDBus++ refactoring already in progress.

Start at the openvpn3-indicator credential query and Cancel flow, using openvpn3 session-start as the reproduction entry point. Verify the session minor status before disconnecting, and observe state changes while the prompt is open. Done means Cancel no longer disconnects a session whose credentials were supplied on the command line, and the prompt closes when the session reaches another CONN_* status.