Dehalfops a selected nick on a channel. If nick is not given,
it will dehalfop you. If channel and nick are not given, it will
dehalfop you on all channels you're on, provided you have the
rights to.
By default, limited to AOPs and those with level 5 access and
above on the channel, or to HOPs or those with level 4 and above
for self dehalfopping.
