m_channames Call ValidateChans() from cull() on unload

This ensures that if ValidateChans() removes everyone from a channel the
channel is added to the cull list at the right time, that is, before the
destruction of objects begin.

The core IsChannel() is less strict than us regardless of our
configuration (for now) but nevertheless this is the correct behavior.
m_channames Remove temporary vector from ValidateChans()