This change modifies extb_channel to allow matching users in secret
channels, which prevents trivial ban evasion by setting the target
channel +s. Information leak due to this change is unlikely since the
attacker would have to know that the target channel exists, the name of
the channel (or guess it), have a specific user they wanted to know
whether was in the channel (and not know already), and the target user
would need to have something like autojoin-on-invite enabled (or any of
the other various ways hostname cloaking is attacked).
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