Auth hangs if script fails to exec

[caius]

1. Configure the plugin to call a script that doesn't exist
2. execve call will fail and log it's failure to exec
3. Currently openvpn will keep the auth pending until the timeout for deferred authentication is triggered, at which point the auth will fail.

We know the authentication can't succeed once the exec fails, it would be nice if the plugin error handler for execve wrote an auth failure to the auth_control_file so we failed quicker.

[g00nix]

It also randomly happens that the auth can not exec the script, so the login will hang at:

Mon Oct 14 19:38:38 2019 us=424660 81.180.227.170:47634 PUSH: Received control message: 'PUSH_REQUEST'
Mon Oct 14 19:38:43 2019 us=563493 81.180.227.170:47634 PUSH: Received control message: 'PUSH_REQUEST'
Mon Oct 14 19:38:48 2019 us=700823 81.180.227.170:47634 PUSH: Received control message: 'PUSH_REQUEST'
Mon Oct 14 19:38:53 2019 us=730106 81.180.227.170:47634 PUSH: Received control message: 'PUSH_REQUEST'