Added ssl::bump_mode logformat code to log SslBump decisions.

Added ssl::bump_mode logformat code to log SslBump decisions.

Reshuffled (a little) how the bumping decision for CONNECT is made to
streamline the code. This was necessary to consistently distinguish '-' from
'none' logged modes.

Do not evaluate ssl_bump when we are going to respond to a CONNECT request
with an HTTP 407 (Proxy Authentication Required) or with a redirect response.
Such evaluation was pointless because the code never bumps in such cases
because, unlike regular errors, these responses cannot be delayed and served
later inside the bumped tunnel.