Support asking for passwords again

Describe the bug
With MFA on the client side, it seems that OpenVPN only asks for a password once. If that fails, it just accepts that the authentication is invalid and doesn't ask again. Would it be possible to support sending a signal or some other mechanism to have a running instance ask for a password again?

To Reproduce
Have an MFA setup. I'm using a systemd-launched instance on the client side. It uses systemd's password authentication mechanism. I have a PR (#649) to remove the timeout, but even without it, if I get it wrong (e.g., enter a code just as it is invalidated or make a typo), I am never asked for the password again. It'd be nice if it would do so after a timeout (just like it does after the system comes back from being suspended and reconnecting) or I could send it a signal to prompt for authentication again.

Expected behavior
When asking for authentication information, ask again if something goes wrong.

Version information (please complete the following information):

• OS: Fedora 40
• OpenVPN version: OpenVPN 2.6.12 x86_64-redhat-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] [DCO]
• Repeat for peer if relevant

Review the client-side authentication flow and the timeout-removal proposal in PR #649, using the systemd-launched MFA scenario as the starting case. Compare it with the existing reconnect-after-suspend behavior. Done means a failed authentication can trigger another password prompt through the requested timeout or signal mechanism.