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.
ListClients: add identifier column
Update Csocket to b5185dc

> Fix IsConnected() to return false when Disconnected() gets called
Replace CIRCNetwork::FindClient() with FindClients()

FindClient() is not enough, because there are no restrictions to used
identifiers. They don't necessarily need to be unique, and the same
identified client might re-connect meanwhile a ghost connection is
still hanging there.
Merge pull request #752 from jpnurmi/findclients

Replace CIRCNetwork::FindClient() with FindClients()
Merge pull request #751 from jpnurmi/csocket

Update Csocket to b5185dc
Merge pull request #753 from jpnurmi/listclients

ListClients: add identifier column
Merge remote-tracking branch 'upstream/master'
Added instructions to remind me how to sync with upstream repository.