Prior to this, m_ison would report a nick as being online if a client
that was not yet registered had chosen this nickname on the same server.
This change adds a check to make sure the struct Client has a
struct User associated with it, i.e. registration has occurred.
Also fix up some return values and stuff to use bool (or void if
nothing). I just did it whilst I was here.
According to jilles, the return value used to signify whether or not the
client had exited. This was error-prone and was fixed a long, long time
ago, but the return value was left int for historical reasons.
Since the return type is not used (and has no clear use case anyway),
it's safe to just get rid of it.
