check exponential backoff on receiving AUTH_FAILED

[cron2]

Observation: a 2.5.6 client connects to a server that refuses entry (due to "account locked out")

Dec  9 19:48:54 hilbov ovpn-medat-ma-tun99[875]: client-XX/2003:d1:2:3::4 SENT CONTROL [client-XX]: 'AUTH_FAILED' (status=1)

will keep reconnecting every 5 seconds - for many hours (auth-retry nointeract in the config).

I think this should be subject to exponential backoff, exactly as "tls failure" or "connect failure". This just fills logs.

It needs to be tested if this is a 2.5 thing ("upgrade client") or 2.6/2.7 behave the same way.