is_immune tests designates to plugins they should always skip processing.
That's typical for naughty connections, but this change provides the ability to handly naughty connections differently than (whitelisted/relayclients/known good) senders.
append optional log_mess to log entry (more description)
subsequent attempts to set naughty don't overwrite the first
set the naughty rejection type to be the reject type of the plugin that marked the connection naughty
get_reject_type can be passed an explicit default
