Revise CChan::JoinUser() & AttachUser()

The old AttachUser() that sent JOIN without topic or names replies
would leave clients in incomplete/broken state. JoinUser() was doing
two things; depending on passed arguments it was either joining user
on the channel on IRC, or attaching clients (properly). Now JoinUser()
joins the user on IRC, and AttachUser() attaches as expected from the
method names.