Using non-TLS-on-connect against TLS-on-connect port should error cleanly, or handle #9
Description
At present, using smtpdane -srv submission $DOMAIN without also specifying -tls-on-connect will result in a hang.
(The -submissions shortcut implicitly sets the -tls-on-connect flag.)
We should handle this more gracefully. Hanging forever is not acceptable.
It is acceptable to spot the TLS-on-connect and issue a warning and continue.
It is acceptable to spot the TLS-on-connect and error.
It is acceptable to just have a timeout over the whole handshake and abort when that is exceeded.
At present, we do have the -connect-timeout flag, but that doesn't apply once the connection has been established.
Spotted while investigating #7