[MINOR] add options dontlog-normal and log-separate-errors

[MINOR] add options dontlog-normal and log-separate-errors

Some big traffic sites have trouble dealing with logs and tend to
disable them. Here are two new options to help cope with massive
logs.

  - dontlog-normal only disables logging for 100% successful
    connections, other ones will still be logged

  - log-separate-errors will cause non-100% successful connections
    to be logged at level "err" instead of level "info" so that a
    properly configured syslog daemon can send them to a different
    file for longer conservation.