This has a separate enabling option channel::channel_target_change.
It applies to PRIVMSG, NOTICE and TOPIC by unvoiced unopped non-opers.
The same slots are used for channels and users.
(resv, cmode +m, cmode +b, cmode +q, etc.).
This is only checked for local users.
For optimal compatibility, a failure for this reason still
returns ERR_CHANOPRIVSNEEDED.
Side effect: normal users cannot change topics of resv'ed
channels, even if they have ops, just like they already
cannot send messages. This only matters if resv_forcepart
is disabled, as the user would have been removed from the
channel otherwise.
