When a user receives a private message, notice or RPL_UMODEGMSG,
add the source to a special set of 5 target slots.
These slots are checked in the normal way when sending messages,
allowing a reply without using up a free target.
This feature will not be very useful if a user is being messaged
by many different users; to help this, messages blocked entirely
by +g or +R do not affect the targets. CTCP replies also remain
free in terms of targets.
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